Back when I was wearing partial units I had a hard time getting a consistent attachment. I attached from front to back, and inevitably the unit would end up a bit off center or too far forward or back most of the time.
When I switched to a full cap I began attaching from the back to the front which allows me to get an accurate attachment 98% of the time.
I attribute this to the fact that when I attach the back first, it gives me a long, straight attachment point that "anchors" the unit to my head and doesn't allow much room for the attachment to go off course.
All you have to do for this sort of attachment is to apply tape to the nape of the full cap, place it on your head, line it up exactly where you want it, then flip up the back and remove the tape liner before pressing the tape onto your scalp. As long as the full cap doesn't move around while you are flipping up the back edge for your attachment, it is very easy to flip the unit back onto your head and mark your hairline and temples in the front for your glue line.
Once that is done flip the unit back off of your head and let it hang from the nape. At this point it looks kind of funny having this thing hanging off of your head, but fortunately no one is judging you for style points so don't sweat it
At this point I apply tape liberally to my scalp to give a strong attachment and to keep the unit from sliding around on my head and irritating my scalp. I apply glue to the hairline, temples, and either glue or tape above my ears. When the glue is dry I slowly roll the unit back onto my head, working from the center outward to avoid wrinkles. When I get the unit to the top of my head, right before it wants to flip down onto the temples and hairline area, I stop.
Next I grab my spray bottle and mist my exposed scalp with alcohol or lace release, then I quickly flip the hairline down and use my thumbs to line it up with the glue line. Once the hairline is in place I flip the temples down and use my thumbs again to feel for proper placement. If anything is out of whack I have a minute or so to reposition before the alcohol evaporates and the glue begins to set.