Cut the opening for the door size per instructions or up to 3/4 inch higher and an inch or more wider that what the actual door frame measures(OD of jamb to jamb, not OD of brick molding)(Remember, if it has brick molding attached then you only have about an inch to an inch and a half on each side and the top for overlap). You are probably going to need a couple studs on both sides of the door and a 2by8 header above. See if you can get those in without having to cut and replace the exterior siding and interior walls otherwise you have a lot of patching to do and may not like the results. Then your trim will go right over without any siding and wall work.
Watch out for the wall thickness. The doors are a standard 4 9/16" jamb made for a standard 3 1/2" studded wall. Lath and plaster and/or new siding means you have to add a trimmer piece to build out the jamb to match. Find a lumber yard with a planer to plane some jamb stock if that is needed.
Look at the floor when you get the old wall out. Put in a metal drip edge such as gutter edging. Caulk 2 or 3 rows along the bottom and set the bottom of the door in first and lean the top in. Even though they are heavy, always dry fit the door and check everything before putting it in finally.
If the door has brick molding on the side and top, then run a bead of caulk on them. Install some J channel above the door for a drip edge along the side.
Shim the door normally and use screws or nails to install don't tighten them down until all are in, door is square, and slides like it is supposed to. Measure diagonally to also check for square. When done, I run a bead of caulk around the outside.
In the inside door cracks, stuff insulation or use non expandable foam to fill the crack and not expand the jamb to the point it causes the door to hang up. A little foam goes a long ways and takes time. Spray a small bead to the back of the jamb all the way around. Wait a half hour or more and try another small bead. Wait and do a third if necessary. It expands alot more than you think and more than the can says it will. It is sticky and messy when you've walked away for 5 or 10 minutes and find it has squeezed out all over your door and jambs, etc.
Then look to see what kind of jamb or work you need to do to make the floor match up. Then install inside trim.