There is currently a method to preserve people who suffer cardiac arrest, called EPR-CAT, where one rapidly cools down the body to give the doctors more time to work before the patients suffer incurable damage due to lack of oxygen. While EPR-CAT currently does not freeze the patients, they are only cooled down to 10°C (50°F), they have shown that the lower the temperature is, the higher chance of survival the patient has.
Experiments on dogs have shown that it is possible to replace the blood with ice cold saline solution as the body does seems to allow for complete lack of oxygen in the blood stream as long as it gets cooled down. The problem, however, is how to rapidly replace the blood with saline solution.
The EPR-CAT technique is currently highly experimental, but show very high promise to increase survival and might, therefore, be the start for proper cryogenics, especially if freeze protection agents are used in the saline solution.
One concern raised in the question is how to deal with organs where the blood does not fully penetrate, such as eyes, and the answer would be plunge freezing. Extremely rapid cooling prevents crystal formation and forces the water to form amorphous ice, which will not damage delicate structures. The alternative, if plunge freezing is deemed to not work well on humans, would be to have very slow freezing with seeded ice formation to allow for more control of which ice structures that forms. Which method that is best will depend on which has the highest chance of success, although plunge freezing would be beneficial as it would allow for a rapid process.
So the way to cryogenically preserve a person would thus be:
- Fully sedate the person (you will not want to be conscious when you get frozen).
- Replace the blood with a cold saline solution with some cryoprotectant agents. This step needs to be quite rapid, so one would probably have to punch holes in either the femorial artery and vein, or the vertebral ones. The solution should be as cold as possible to help cool the body.
- Plunge the patient into a vat of liquid ethane, propane or some science fiction-y solution with very cold properties. Liquid nitrogen does not work due to the Leidenfrost effect, the body would not freeze fast enough and ice crystals would form.
- Figure out a way how to unfreeze the patients safely.
- Realize that step 4. probably should have been considered before step 1.
This is a bit of a tricky issue. As Faulkner pointed out in their answer, we have not yet revived a preserved person. So the short answer is "we have no clue how to safely thaw someone". However, we might learn something from nature seeing that there are ample of animals which gets frozen each ear and then safely thaws. I found a decent overview text written by a researcher on animal adaption for cold environments and will use that as a reference in this bit.
While the preferred method to freeze would be as fast as possible, an animal frozen as means to survive winter will thaw quite slowly as spring arrives. The key issue for those animals is that the freezing process will suck out a lot of the water from vital organs so that the ice formation is around them rather than in them; this requires slow freezing and also slow thawing to allow the water to migrate through membranes. The liquids left inside the vital organs will be a highly concentrated slurry of anti-freeze agents which prevents the organs from freezing entirely. Plunge freezing would go the other way around and cause amorphous ice to form throughout the body and might, therefore, allow for a more rapid thawing as no water need to migrate.
Body functions of someone frozen will be shut down; a frozen person will be technically dead while frozen since the heart will be stopped and no brain activity will take place. We currently do not know exactly how this function in frozen animals, but the signal to stop the organs is probably linked to the temperature of the body or ice formation in some vital region or similarly. This also means that we have no clue what it is that restarts the organs once the animal is thawed. However, for space traveling humans, this should not be an issue. As long as we can thaw the person without causing (too much) damage to the cells, then we can always use heart starters and modern technology to ensure that the body functions awakens properly.
The text I use as reference for this bit also states that animals which survive winter by getting frozen also have enhanced cell repair activated when they are thawed. If we manage to enhance our knowledge in stem-cell technology or just find other ways of how to stimulate and regulate how our bodies self-heal, then we should be able to make something akin to stim-pack drugs which would repair any damage induced by the freezing and thawing.
Soooo.... how to safely do the thawing?
- Slowly increase the temperature in the cryo-pod.
- Once the body is warm enough to allow the solution with anti-freeze in the circulatory system to move easily, then start replacing it with oxygenated (synthetic) blood.
- Continue to slowly increase temperature and time the restart of organs to when the body reaches suitable temperatures. The restart will probably be aided with heart starter and similar medical devices.
- If available, inject stimulants to improve cell repair
- The eyes will likely hurt if they have not been exposed to light for a long time. Administration of sunglasses or keeping the light dimmed for a while in the ship is advised.
- Administer either two Diazepam or a generous glass of whisky to the recently thawed person to help them cope with any mental and physical pain that the process likely causes.
For a cryo-pod in a movie, I guess that the people would walk (Not naked! Thin clothes ought to be best*) in to a cocoon-like chamber and a device would close around their necks. This device would then first sedate them, then replace their blood with the anti-freeze and then the chamber would be showing gas streaming in as it would be tricky to fill the chamber fast enough with a liquid. However, the gas would have lower heat conductivity than a liquid and would not work properly for real freezing. If you're writing a book (or want to make a bit more shocking scene in the movie), the floor would rapidly be pulled down into a vat underneath the chamber and the chamber would be cooled down while the person freezes in the liquid. After a certain set-time, the floor would rise again with the person and s/he would be kept in the pod for the travel. Unfreezing would then simply be to slowly increase the temperature in the pod while the neck device pumps back the (probably synthetic) blood into the person. The final touch would be a heart starter that would extend and kick start the heart before the pod opens.
I would never, ever volunteer as the first person to try the method. Especially not after having practical experience of cryo-microscopy and knowing how often the sample preparation fails due to improper freezing. But, with some time and trials, this would probably be close to how it'll be done.
*I originally stated that one ought to be naked in the cryopod as I thought that the clothes would help vapor film formation and prevent rapid freezing. However, once I actually thought it through, I realized I was wrong and that a thin cloth layer might actually help the freezing.
By adding a layer of thin cloth, the film formation from the Leidenfrost effect is actually prevented as the cloth retains the liquid better than naked skin. The cloth should be very tight fitting to prevent a vapor film from forming under the clothes (if it does form, then naked is better) and should have good thermal conductive properties. That is, tight, thin spandex (preferably with some science fiction-y materials incorporated) would work best.
I do not have any links for this claim, it's taken from the basic safety course I've had when working with liquid nitrogen. Most safety courses only states that "you have to wear a lot of clothes", although those with experience have taught me that "if the clothes are not thick and/or loose, then no clothes is safer".