The legal requirement is to do it safely. If you're not sure, it's a smart idea you have there, to get someone knowledgable to do it for you :)
Setting it up properly can be a black art too, pro LPG fitters have a world of experience to draw on.
You must tell your insurers (for a road vehicle) who may or may not want some kind of certification, companies vary. If you don't tell them your insurance will be invalid and you stand to lose everything and more.
It's easy to find LPG fitters, google LPG + install + [insert your county] but be warned not all will want to finish off a DIY job.
good luck, Ross K.