Powerwash, then powerwash again. Check all the ledges especially in the wheel arches as the mud likes to hang up in there. in essence get rid of the mud first.
Any heavily rusted chasis/steel parts would benefit from wire brushing and acid washing to kill the rust - waxoyl claim that their product negates the need for the latter but they're thinking about MG doors etc.
Brush or roller paint the chasis with smoothrite if you want - the finish isn't critical as the subsequent waxoyl will cover it.
Spray the inside of the arches with a stoneguard type of paint - not underseal.
Once you've sound surfaces cover them with waxoyl, get the stuff into all the hollow sections, such as load space stiffener U sections, chasis outriggers, chasis rails cross members etc etc. Use plenty of it, but avoid the exhaust
Halfrauds sell the pressure canisters & spray nozzles & lengths of pipe for internals. But if you have a compressor use an underseal gun as the canisters need constant attention to maintain a decent flow & coverage pattern.
For some reason the clear, without bitumen, version seems to cling better and last for longer.
Finally when you've finished, you'll need to arrange a day to come & do mine