Instructions
Log in to your RuneScape paid member account.
Open your bank and withdraw a pestle and mortar. If you don't have one, you can purchase this item from the herblore store in Taverley, or from the Grand Exchange.
Withdraw a desert goat horn or a billy goat horn for every goat horn dust that you wish to make. You can get these by killing desert goats or billy goats, which are located southeast of Shantay Pass and west of the town of Nardah. Occasionally goat horns are available for purchase from the Grand Exchange.
Click on the pestle and mortar in your inventory.
Click on a goat horn. Select the number of goat horn dusts that you wish to make from the menu, if you have more than one horn. The goat horns will be automatically crushed and replaced by goat horn dusts in your inventory.