To be honest with you, my attachments have been 10x more consistent with a full cap than they were with a topper, even from the first full cap I ever attached.

I think I got lucky and found a method that works well for me on the first shot, and I have used it every time since (sometimes with minor mods)

I won't go into 100% detail, but I start in the back taping the piece at the nape. Then I set the unit right where I want it and flip the back edge up to peel the backing from the tape, then I tack it down in the center and move outward in both directions. Once the back is in place the piece will naturally fall into place in the front. At this point I usually roll it forward and mark a dotted glue line with a blue lipliner pencil at the front from sideburn to sideburn.

Then I flip the unit back and apply tape above the ears and up the temples, and I put a contour strip at the hairline, keeping everything back about 1/4" or so. Then I use white glue all along my temples and hairline, including on top of the blue liner or walker tape to kill any shine.

Once two coats of glue are applied and dry, I roll the piece up to just in front of the crown so it sits on the top of my head with the sides flipped up like wings. A little spritz of alcohol over the adhesive area, and then I attach the hairline quickly, followed by the sides and temples. The alcohol is the key so you can get it just where you want it.

My motto is "Tape for strength, glue for finish". I use blue liner so the piece isn't going anywhere, and white glue for a clean finish.

All you have to do is take your time, and this method could also be used front to back as well.