are generally easy to install, sometimes the hardest job is to remove the old door and frame. Or to get to the rough opening.
Remember, the opening will not be square, but the new door kit has to be. You do this with shims and adjustments here and there to get the kit in square.
I like to screw the jambs in, this way the screw becomes an adjustment tool.
Shims come in pre packs at home centers.
The kit should be about two inches smaller than the rough opening on the sides. I like to install more shims on the hinge side than the lock side. This way the throw of the dead bolt will reach into the wall stud for more security.
Before sealing it up, make sure all four sides of the door are properly seated in the frame and opens and closes smoothly.
What ever you do, do not do any sawing or sanding or shaving of the door itself. All adjustments will be with the frame which adjusts the door.
more shims on bottom of hinge side makes top go in. Study your old geometry before tackling this