First u need a talisman/tiara/talisman staff (air talisman/tiara/talisman staff, fire talisman/tiara/talisman staff etc.), then u need to go to altar, for example air altar is located west of varrock: Check this map. Then use ur talisman on the mysterious ruins, or if u r wearing tiara or wielding talisman staff, just simply press mysterious ruins with ur cursor. Then you will be teleported inside, then just simply press air altar with ur cursor, and u have as much air runes as u had rune essences in ur inventory. U can also use pure essence, which can only be mined by members, but f2p players can buy it from Grand Exchange.