Screen printing t-shirts on just one side is pretty easy. I tell you how to do it in my pages here. But screen printing a t-shirt on both the front and the back requires a few more steps.
First I will print one side of the shirt. It doesn’t matter which side. Then I will lay the shirt out to let the ink dry completely. Later, maybe even the next day, I will print the shirt on the other side. It is probably possible to print both sides in short succession, but it would be hard to keep the wet ink from smearing, or coming off on other shirts. I suppose if you have a flash dryer, you could dry the ink enough so that it would be safe. But that’s more of a commercial type operation. Flash dryers are expensive. So print the once side, let dry, then print the other side. Let it dry.
Now that both sides are printed, you need to heat set the ink. I do my heat setting in the oven. When the ink gets hot, it gets soft, and can come off on other stuff. So what I do is lay a clean piece of paper over the dried design on one side. Usually the smaller printed part. Then I will fold the shirt and leave the other printed part face up, not touching anything. This way, if the hot soft ink transfers a little, it will only be to the piece of paper, not to another part of the shirt.