Parts may well already be in primer, check by comparing the inner hidden side, to the outer visible side, if there is a difference in colour/texture, then this points to the part already being primed, so just gently key the primer with Grey Scotchbrite and soapy water, degrease, base coat, then lacquer as normal...
If the part needs priming, then use an aerosol plastic primer (Halfords) then, if the parts are of high quality plastic, and free of defects, you can paint straight over the plastic primer, as above. If the plastic isn't top quality, then it will be necessary to use a high build aerosol primer, OVER the plastic primer... this should then be carefully flatted with flatted with 1200 wet/dry, then base coated, and lacquered...
Easy job really

Thom