Today I did some plying.
For small amounts of handspun singles I wrap the yarn on my hand (Andean method) and then carefully slip it to my wrist and make a plying bracelet. Holding the two tails together I ply the singles in the opposite direction it was spun to create a 2-ply yarn.
For larger amounts of yarn I prefer to use the Handy Andy plying tool:
Wrapping the tool
Plying on the EEW 6.0 electric spinning wheel