First, complete the quest "Smoking Kills". Then complete slayer tasks that were not assigned by the master in Burthope village. You'll get "Slayer points" as you do them.
When you have 400, go to a slayer master, right click them, and choose "rewards" go to the "learn" section, and choose the slayer helm option.
Now get a facemask, a nosepeg, earmuffs, spiny helm and a black mask in your inventory, and use one of them one one of the others. You now have a slayer helmet.