How do you re-clify a vehicle?

i was wondering how to re-clify a vehicle so that it is cled as a light Weight agricultural vehicle. now i know that the vehicle has t be adapted in such a way to have use from a agricultural,horticultural or foresty view and weigh less than 1000kg, but how do i re-cl it? and des it the need to be checked by the DVLA or vosa etc?...thanks sam.