At 18 crafting, you use a silver bar with a furnace while having a holy symbol mound in your inventory. This will get you an unstrung symbol. Then, get shears and shear a sheep. Use the wool on a spinning wheel to get a ball of wool. Now use that with your unstrung symbol. Finally, bring your symbol and talk to Brother Jered in the Monestary (requires 31 prayer).
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