I recently replaced the front door on my house. It wasn't too bad. You
need to remove the entire old door and casing. After that measure the
opening and get a door that will fit with at least 1" clearance on the
top and sides. (or the same size door you are removing)

then stick the door in, make sure it's level, and add shims to make up
the space between the door and the jack studs. Filled the gaps with
spray foam, then installed casing over it.

The door I replaced was a little taller than the new one I bought, so I
had to but a 2x4 on the top to close the gap.

Your local huge depot should have a door w/ instructions on how to get
it done.