recon gear box

hi all I need to change my gear box, I have another box to put on  but I want to recondition it before I put it on, does any one know what i will need or any good guides to get me started.
Its for my 86 1.8GTi 8v Cab

Seller on Ebay [search it] sells gearbox kits , most buy one rebuilt / reconditioned its not an easy or cheap task