If you need to see how to create a Custom Field then please click Here
Click on Products
Click on Manage Products
If you are adding the custom field to a new product press Add Product or if you are adding it to a existing product find the product you want to add it to and click the pen and paper icon.
Click on the Custom Field tab on the top menu.
It will now display all you the Custom Fields you have already created, Click HERE for the guide to create Custom Fields.
All you have to do now is fill out the value of the field e.g. Size and the value could be small or set a currency/ different price
If you've added the information you wanted and all of the fields were filled in correctly you're done. Now all you have to do is apply them, to see how to apply them to a listing template please click Here