Start at the Outpost North-West of West Ardougne and talk to Jorral. Receive the Enchanted key notes (and the Enchanted key if yours has been destroyed or dissolved). Allow Jorral to deposit all your items (except the key and the notes) in the bank, as you cannot use the key to enter the past with any other items equipped or in your inventory. If your enchanted key happens to be on a steel key ring, simply proceed talking to him and the key will automatically come off the key ring and you will need to still deposit it.