I would set them in your raised beds, you want to catch the divvils that are there and not attract more!
I don't know what design of slug trap you have but "Beer is best" stale beer even better. If you have the type you sink into the soil, leave the lip protruding above the ground about one inch (don't ask what mm or cm)! This is so ground beetles which are benificial to the garden don't fall in and drown.
Cheers, Tony.