Edit that particular item using item editor. Next open your textures/gui and look for the icon of that item. Go back to your item editor and press Shape of Inven and select the .dds where the icon of that item is. Last, put the inventory icon values, it is the location of the icon on the .dds
Take a look at my screenshot and its coordinates
If you want to have the icon of WHITE I LOVE YOU SOCCER SHIRT, just put 12 2 in your Inventory Icon