Well, actually, it's not that easy. Usually you'll have to readjust the hinges on the door. The problem with that is the doors are pained with the hinges on. If you've lucky the hinge locations will match otherwise you'll need to shift the door a bit to get the door to close properly.
There is one or two bolts on the hinge. I know YJ's have one for sure. Not sure aobut TJ's. Remove the nut then open the door at least 90* then lift up and wiggle.
Full doors are heavy. You'll want to put the door on the same way you took off the half door. Then see if it closes OK. If not then you'll need to loosen the 6 torx bolts on the hinges. Then the door will flop around. It's best to have two people. One to lift the door and another to tighten the hinges once you get the door closed. With the door closed you'll want to roll down the window if it's not already and push up on the window sill at the top to lift the door a bit. Then tighten the bolts. Reattach the mirrors, etc... (BTW: one bolt will be under the mirror.)