First purge the slugs by feeding them for five days on a single-item diet of lettuce, cabbage or carrot peelings. Then starve the slugs for two days more to remove any remaining digested matter in their systems (this step is optional, but if you don't do it, you will have to gut them).
Put the slugs in the freezer to kill them. Make up a solution of equal quantities of vinegar and water and soak the frozen slugs in it for a few minutes to begin drawing out their slime.
Put them in a pan of cold water with a dash of vinegar and bring up to a simmer. More slime will come out. Drain the slugs and rinse them under the tap, removing the slime with your fingers.
Repeat the simmering and rinsing, with fresh water and vinegar each time, at least three more times, until no more slime comes out.
Put the slugs in a pan of fresh water with some sliced carrot and onion and a bay leaf. Bring to a simmer and cook until the slugs are tender - probably just 5-8 minutes. If you need to gut the slugs, make a slit one-third of the way down the back of the body and remove the intestines.
If you read it all then congratulations you utter psychopath.