It's easy if you know what you are doing, like all things really.
Good advice above though don't diy unless totally confident, there 's only a few wires need shielding, this is due to proximity reasons to high voltages(ht leads).
The best wire to use is type 44, the guy I use has been doing these before most on here were born, and gives a lifetime warranty on all his looms, has made in all forms of motorsport inc F1 where Ferrari even x ray the connections!!
He;s made some very "radical" looms as well, he does mostly TVR, as the insulation is the favourite "treat" of mice who chew through it on a seriously regular basis on tvr cars.
And tvr's are the biggest pita to fit looms into!!
tabetha