As Shadowzee said, if you want to use giant snakes, you'll have MANY hurdles to cross. So, instead of saying all the reasons why you can't, I'll essentially give you the notes I used to figure this out for in my own writing. (Using Shadowzee's order for simplicity.)
Saddling - Snakes have scales. They shed their scales about monthly and present various signs they are about to do so. Keep an eye out for the signs and wait for that time of the month to subside and you should be fine in that regard. Assist the snake with the uncomfortable shedding process, help it get sufficient food in that time, and so forth and you can expect it to be a fairly loyal and reliable ally. As for the outright saddling of the snake, it depends on a couple factors. First, are the scales still small or are the proportionately large? If they are large scales, first, congrats: your warriors have a free source of armor that they are given every month. As this question's answers point out, the armor isn't perfect, but as a free resource, your people shouldn't complain. Second, you can design the saddles to slide under and latch onto the scales. While this would likely be uncomfortable for the snake (like something getting under your fingernails), it's something that they could get used to in time, not dissimilar to a horse's shoes. If the scales are small, you'd then have to consider if you're willing to hurt the snake or if you're insistent on being as humane as possible. If you don't mind hurting the beast, you can always have the saddles affixed to the serpents via spikes. For a more humane method, you could have bicycle-kickstand-like features coming from the sides designed to serve as a corrective apparatus to keep the saddles from sliding to far in either direction, but this would have to be a long, slim piece of metal that would likely be fragile, presenting it's own disadvantages. You could potentially design the saddle and bit to all be a single unit, therefore making it so that the saddle stops itself in that way from sliding too far in any direction.
Riding - As for dealing with the constant swaying motion and snake-sickness, the only way to get passed this is to make it something a portion of the snake riders have developed. Logically, it'd be a result of evolution. Being able to ride the snakes would be seen as a great skill and any who could do so would be sure to find a mate in their tribe and the gene would pass along to the next generation. Due to the risks of dealing with the snakes, there wouldn't be a guarantee every snake rider would live long enough. Snakes are dangerous, fickle creatures. If they don't want a rider, they won't accept the rider. Humans are just as much food as anything else is in their eyes. Finish this by making the resistance to the nausea-inducing effects of riding the snakes a recessive genetic trait, and you keep your tribe's ability to ride snakes be a little unpredictable and even more valuable. As far as the tribe would be concerned, this ability would be something that would be considered a gift from the gods or ancestors as it would allow these snake riders an indisputable advantage in war against other tribes and give some benefit when hunting, so long as the snake works with the rider and not against them.
Cold-blooded - It would make sense due to other factors (namely their size) that they'd rather be active during the day when they can be in the sun, but I'd argue they'd hunt in the early morning and later in the day while sunbathing in between hunting sessions. Additional natural heat sources in the environment such as rocks that naturally store heat longer would probably be a good thing for the serpents at night. That said, this doesn't mean snakes have to be active during the day and inactive at night. There are both diurnal AND nocturnal snakes. In fact, it may be more accurate to consider snakes as crepuscular as opposed to diurnal or nocturnal, but that's a question for a different SE.
Ambush predator - Quite simply, this can be gotten around by having the snakes raised by the tribe be bred based on willingness to hunt and work with the humans. You could have ambush snakes setup around the village as a defense measure, but the ones most likely to survive into the next generations would be the ones that display a greater interest in hunting. The problem with this: snakes are lazy (read: efficient) by necessity. More movement means more energy expenditure and risk of being predated. Snakes thrive off of acting based on need more than demand. If they need warmed up, they'll move to somewhere they detect heat. If they need food, they'll try to lure prey first, then hunt if they can't catch an easy bite. When they do get food, they prefer larger prey so they can spend longer time without needing to hunt. (I noted in my own info that snakes are very Boolean which makes for convenience when writing.)
Food - The snakes will more likely see humans as an analogous prey to how real world snakes see mice. Humans would be small prey that can't provide enough energy on their own, but a snack is better than going hungry. That said, if snakes find cooperation with humans to be of benefit to them, then they will likely work alongside humans to find food for both. This will help if your story has sufficient enough prey for the snakes available. If you're in a low-density area, the snakes may see humans as being more useful as food. As a result, your peoples will likely give the snakes first picks at food before hunting for themselves. Additionally, depending on the species the snake is based off of, it's not unlikely the snakes may even eat each other! So be sure to keep that in mind when detailing these creatures. Your humans would likely know these aspects to their snakes' behaviors and will more likely than not do what they can to keep the snakes fed at minimal risk to human life. Alternatively, humans who have committed crimes or are prisoners of war may be used as sacrifices to the snakes as a way of keeping them appeased. It's not unlikely your people would even see snakes as protective deities or even minor gods, in their own rights. (This would be akin to Hindu mythology in that respect.)
Cooperation - The only way to make sure the snakes don't escape or eat the humans is by making sure the humans keep the snakes well-fed, well taken-care of, and well-groomed (verb definition 3: "to get into readiness for a specific objective"). Snakes will do whatever is less work for themselves. This isn't a bad thing, though. Your humans would be used to this and would already have safeguards ready for it including storehouses full of captured prey that trappers would have caught, ready to give the snakes at any time. The reason why these prey may be given to the snakes instead of eaten outright could be a variety of reasons: the animals (likely mice) taste pungent and gamey so it's considered a last resort, your people hold a reverence for the snakes or may be fearful of the snakes and use a symbiotic lifestyle to reduce the risk of becoming prey, or your people may see snake riders as being capable of hunting game much larger and much more difficult/dangerous to catch than what they could normally get (possibly thanks to the venom of the snakes) and see the reward as being well worth the risk as a trapper may be able to get enough low-quality prey for the tribe, but the snake riders could get enough higher-quality prey while also keeping a potential predator subdued and subordinate.
Position on the Food Chain - In this world, humans would be pretty much the lowest animal on the food chain if EVERY animal is enlarged. It's sort of like how it's a wonder humans even survive the Pokemon world. (Two different links.) Bugs would on average be the size of house cats. House cats would be the size of lions. You could expect humans to be the prey most predators try to eat because they'd be the easiest targets based on size. Even mice would be the size of large dogs or small horses, so you can expect humans to need to rely on their wit to survive more than ever. Also, just as birds normally target mice in our world: humans in this world will need to be on constant lookout for feathery death from above. This may actually be the thing that solidifies a mutual relationship for snakes and humans. While the snake kills prey for itself and the tribe, the human riding it could serve as a defense system against birds looking for an easy meal. If the human kills the bird, then that's an easy day's haul.
Training Them to Maneuver With a Rider - This would be the easiest part. The snake would want its defense system. It will adjust its movements to protect the human, and the human would train to lay down when the snake is moving as opposed to sitting up. This would reduce drag, reduce the risk of a predator sweeping the human off of the snake, and make it easier for the snake to move through lower-clearance areas without worrying about losing its rider. The biggest concern would be humans and snakes finding prey within an area. I mean, snakes can eat insects if need be, so that's always an option, and humans have a history eating bugs too. Apparently bugs taste nutty. (In fact, the UN is even encouraging people to eat insects!) So, you don't have too much to worry there, but be sure to feed the snakes first and take whatever is left, or your people may not be in the best position. The snakes would likely not eat the humans outright due to the benefit humans give, like slimes presumably might offer to bigger fantasy monsters.
Of course, this only touches upon SOME of the things to keep in mind, but it should be more than a start. I hope this helps you in the end!
