You all probably have your own way of initially setting spokes to the same thread engagement prior to truing a wheel, but here is mine and the tool I made to do it.

I am a fan of Sun Ringle CR18 double-walled rims and when building wheels with them they present a small problem in that "sometimes" getting the spoke nipple onto the spoke can be awkward especially during the latter stages of the build process when there is less "play" in the whole system.

One of the key attributes of building good, even, and consistent wheels it to start all spokes at the same level of engagement.
To this end I made myself a little nipple-driver that I use to attach spoke-nipples to and then easily screw each nipple on to the spoke to a uniform position.

The tool consists of a spare spoke and a spare nipple that is wound onto the spoke-thread and set with thread-loc (or similar) leaving enough spoke thread protruding to attach your spoke-nipples to by a few turns.

Basically you put the nipple onto the end of the tool, wind it onto the spoke (even through the deepest double-walled rims) and then when the tool bottoms-out, grab the nipple on the spoke and unscrew the nipple driver.

Note: the spokes used to make these are overly threaded with long threads (because I had them), but the same effect can be achieved by filing-down a spare nipple onto a shorter-threaded spoke.

Below is a picture of a bare nipple driver next to on that has been cut down and inserted into screwdriver handle with hot-glue.

They work very well.