Its not hard if you can spray to start with, if not get it done professionaly.
For best results take the wheels and tyres to a garage and get them to pop the top beads off the tyres, remove any old balance weights and remove the tyre valves (Taking care to NOT drop the back of the valve inside the tyre). This way you can easily mask the tyre off to avoid overspray and spray the valve seat also. Once done get the garage to fit a new tyre valve, re-inflate the tyre for you and re-balance the wheels.
Failing that take them to
Peninsular 4x4 (Sprayforce) for a top job done professionally
I guess the cost would be approx £30 - £40 per wheel to be sprayed professionally ??