get a big board, lay your loom out on it, it helps if its warm cos its more flexible, mark out the connectors on the loom and fix them to the board. then lay the loom out in straight lines , if you can, secure the loom down to the board with nails hammered into the board and loops of string tied round the loom. photograph everything. once the loom is layed out, carefully remove the insulation from round the wiring. write on the board an identification for every connector, abc, or 123 etc. then take notes of the wires from each connector colour, length and destination etc. then, assuming you have all the correct connections and wire codes as well as the correct crimpers, then remove the loom, and starting wherever you want lay your new loo out one wire at a time till theyre all laid out on the board, then crimp the connectors on and insert into the terminal blocks, finally, wrap it with the correct wrapping. (or thread the wires through the correct sleeving before crimping them up.
doing it this way has the advantage that you can walk away if you cet bored and come back to it without losing your place. its also worth remembering that if your bike is a bit rare, and its standard, then buy enough materials to make 3 or 4 looms up. and sell the spares on to cover the cost of making them. you should be able to get your own loom for free if you do it this way by selling a few spares.