Actually you don't technically help them mate, you just make a strain that can't reproduce via hermaphrodites self-fertilizing. You keep the males from being out-bred that way.

C. elegans has male and hermaphrodite sexes, not male and female. The hermaphrodites self-fertilize slowly to produce a few hundred hermaphrodite offspring, while mating with a male gives them many times as many offspring with half being male. But the lab-bred males are so bad at mating that even if you have a population that's half male, they get massively outbred by the hermaphrodites selfing, and over a very few generations maleness just falls out of the population. You'll wind up with about 0.1% of the population being male in the equilibrium due to the occasional hermaphrodite egg dropping an X chromosome during development (no Y chromosomes in this species, males just have one X), but they are continually diluted out by the hermaphrodites.

What you do is breed in a genetic change that makes the hermaphrodite's sperm fail without affecting the male's sperm, preventing selfing from producing any offspring. The occasional successful male mating is productive enough that they can still on average replace themselves and their partner and then some, it just has a much longer doubling time and thus when in competition with selfing gets diluted out.

More recent wild isolates can still mate well (and also show a lot of interesting social behavior you don't see in the long-established lab strains) and their populations remain just under half male for a long time. Dunno what happens when you let the two populations mix.

EDIT: and just so you know, I upvoted 'very carefully'

How do you help a nematode mate?

Very carefully. :)