I have recently taxed 2 EV's. I would recommend that you get the car MOT'ed after the conversion and insured. Then go to your local DVLA office with your V5C document, MOT and insurance documents. You also need to bring supporting evidence of the conversion, receipts for major components and pictures also help, try and get one that clearly shows the number plate and electric bits. Also if the car is newer than 2001 (check date, could be wrong) you will need a receipt/letter from the "company" that carried out the conversion. Be creative here if this applies.
With all of the above paperwork the DVLA local office will have to issue a free tax disc. They will take copies of your receipts, photos and send that to Swansea along with your V5C document (for change of taxation class and fuel type). They may ask for an inspection later and will arrange an appointment if required.
It will probably take some time, but be patient, friendly and don't leave until you have your tax disc.
Renewals should be able to be done online or at the post office were they will probably ask, "do you want 6 or 12 months tax?"