To start the Imp Catcher Quest go to the Wizards Tower and talk to Wizard Mizgog. Imps around the Runescape World have stolen Mizgog's beads, he wants you to find him his four coloured beads. Attack Imps (level-5) and they drop bits and bobs like buckets of water or shears. Sometimes they drop the beads you want. You only need oone of each coloured bead. When you have all the beads go back to Wizard Mizgog and he will give you a reward.