#1, That old paint and old caulking needs to be clean off or the base will not sit flat on the wall.
#2, Learn how to coup cut the inside corners.
https://www.familyhandyman.com/carpentry/trim-carpentry/how-to-cope-joints/view-all/
#3, If the gun is set right there should be no need to set the nail.
#4, It's always best to hit the studs for more holding power.
The way I do it, right or wrong is to find two studs to see if the wall was laid out on 16 or 24" centers, once I find the center of a stud I mark it and pull my tape measure out and lock it laying flat on the floor with the end of the tape on the mark and start nailing the base at the 16 or 24" marks.
That way your not wasting time measuring for each stud.
#5, One nail at the bottom low enough so the 1/4 rd. will cover it up and another one 1" down from the top
#6, Every other stud should hold it, but I go back and look for big gaps and push on it to check for flexing, if there is any I push it in and nail it at a slight angle for more holding power.
#7, There always will be small gaps at the top that can be filled with latex caulking, when caulking always only use just enough to fill the gap, there's 0 reason to have it all over the wall or the face of the base.
I use Fast and Final when filling the nail holes, once again you only need enough fill the hole, not all over the face of the base. It drys fast with no flashing when painting.
I use one of these when measuring to cut the outside corners.
http://www.rockler.com/starrett-7-p...3fcHnn4p_-9LGmOfLfpcbfZnbnKgVsFgaAtL_EALw_wcB
(outside and inside corners are almost never an exact 90 deg. angle.)
When measuring for cutting the short piece at the end of a wall it's best to install the long pieces first.
I find it works best to over cut the angle by a few deg. so just the outside tips are touching so there is no gap on the outside edge.
I also would much rather have the piece a tiny bit to long and have to gently tap it in then to have it too short and have to deal with the gap.
Often times there's a tiny tip that will stick out that can be sanded off.