Instructions
Buy a Dragon Full Helm armor set from the Grand Exchange in RuneScape. Only this type of armor or higher can be trimmed. Anything lower than Dragon will not allow you to trim it.
Slay a high-level monster in Level 3: Treasure Trails, which will make it drop a clue. Use the clues to solve where the hidden treasure is. One of the prizes for hidden treasure is a gold-trim ornament kit, which allows you to trim your armor.
Ask another RuneScape player if you can buy one of his spare ornament kits. Make sure you buy an ornament kit that matches the armor you want to pair it with. A Dragon armor set can only be paired with the Dragon ornament kit.
Place the armor and ornament kit in your inventory. You can't trim armor while your avatar is wearing it. Click on the kit and use it to trim the corresponding armor.