The technique I read, probably in a Bicycling article on wheelbuilding, was to use a Philips screwdriver. Put the shank just outside the radius of the hub flanges before the first cross, over an outer spoke and under an inner spoke. Gently lever it until the outer spoke lies flat. Repeat around the wheel.

Another part of the procedure in the same article was to use pliers to bend the spoke just above the nipple so the nipple comes out of the rim as straight as possible rather than some extreme angle and poorly supported by the holes, but I haven't seen that recommendation repeated many other places. Does anyone around here actually do this?