I'm building my first set of carbon wheels (Chris King ISO SD 100mm front hub, ISO B 148mm boost rear hub, Derby AM 40mm 27.5" Carbon rims, 32 hole 3 cross 14/15 double butted spokes to 14mm brass nipples).
Dropping a nipple into the rim is a pain, it's difficult and annoying to jiggle it out and it only gets harder as more and more spokes are inserted.
I made this handy tool from an old spoke.
Put the head of the nipple on with a couple of turns and you can easily guide the nipple through the rim and connect with the spoke you are inserting on the other side. Then you can grab the nipple, unscrew the tool and insert your nipple driver to screw the nipple onto the spoke. Easy peasy.
For my second wheel I realized I didn't need any torque at all and spinning the tool was the most important thing. I came up with this which is even simpler. The grip is made from gaffer (stage) tape wrapped around the spoke a few times. It worked really well.