You need level 50 Runecrafting to enter the Runecrafting guild -- which is on the (second?) floor of Wizard Tower, that place between Lumbrige and Draynor.
Then you play the Great Orb Project minigame from within the Runecrafting Guild -- by participating in the game, you get runecrafting guild tokens, which can be exchanged for items, including the robes you want. =)
The game isn't too hard, you're placed on a team with a few other people: either green team or yellow team. The minigame will teleport you to different rune altars, where green and yellow orbs will appear. Your goal is to attract your team's colored orbs to the center altar.
The team with the most points wins. There are 6 rounds ftp.
Even if you lose every round though, you still get some tokens. The games not too hard; you'll get the hang of it.

