Simply head to www.skiddle.com/orders and click 'Request ticket resale'. Once you've done this, you'll you'll be asked to enter the last 4 digits of your payment card for security.
Alternatively, if you have the Skiddle app then head to the 'My tickets' section and under the 'Options' click 'Re:Sell'. Next, you'll select how many tickets you wish to put up for Re:Sell and for added security, you'll be asked to confirm the last 4 digits of the card used to make the purchase in order to proceed. (Please note: if you have paid using Android or Apple pay then just enter the last 4 digits of the card you have linked to this)