How do worms mate?

How much detail do you want? There are many species of worm so which do you ask about? Lumbricus terrestris* (Common Earthworm) is hermaphrodite: It quite normally has both female and male sex organs and female and male gametes. Reproduction involves two worms, neither of which have a penis. Sperm from one worm is mixed with eggs from another and these are left together in a egg cocoon in the soil. There is no internal fertilisation. Their reproduction is sexual, not asexual.

*If YA allowed formatting this species name would be in italics.

They produce a band of slime around their genitals and move close together, then their gametes swim through the slime and join.

Male worms have little tiny worms called worm whangers that they insert into the female worm's poon and ejaculate worm juice. 

ordinary rainworms are not asexual. Every individual has male and female parts. they bang eachother simultaneously