A nature tiara is made by combining a Nature talisman with a tiara. You need 44 runecrafting to use it. In order to combine it with a talisman, simply use the tiara on the Nature altar with a Nature talisman in your inventory. Doing so earns you 45 Runecrafting experience. You can also get a nature tiara as loot from a magpie impling.
A nature tiara can be worn in the head slot, allowing you to left click on the altar to enter, saving you an inventory space.