The more stable a platform you can provide for your forearm, the better you can stop it from "wobbling".
Bracing with the other arm could help.
Lying with your feet toward the enemy and bracing against your legs might help even more.
(Look up the Creedmoor position and the "dead frog" shooting position, for example.)
But you might decide that creates too much risk of hitting your own legs with your own energy bolt.
If the direction of the bolt is determined by the alignment of the bones in your forearm, you have the problem of aiming, because your forearm is not equipped with gunsights and it is hard to get your eye in line with it.
You can use something like the stance in Hueco's answer to press your upper arm against your face and get your eye close to the axis of your forearm.
You can also lie down at an angle to the direction you want to "fire" and press your face down on your shoulder to get your eye nearly aligned with your forearm, placing a hand under your arm to brace it against the ground.
But what about adding artificial sights? If you clamp a sighting device to your forearm, will that interfere with making the magic bolt? What if you have a surgeon graft ridges of bone onto your radius so that you have raised points on your skin that you can sight along?
Regarding the doctrines of facing the enemy in order to minimize the number of organs hit by one shot or to take advantage of body armor,
consider how the use of magic energy bolts changes the tradeoffs.
What is the nature of how the bolt injures someone, and how can you minimize the risks?
For example, I suspect that part of the thinking behind "only let them hit one vital organ at a time" is that nowadays you might survive being shot in one vital organ. In the 18th century that was generally not possible, so giving your opponent one chance to hit two vital organs at once would have been better than giving them two chances to hit just one of the organs.
But now that you're throwing around magic energy, how much does it matter which way you are facing an incoming bolt?
Can you provide any kind of armor or magic protection against it, and how does that work?