if this is a direct fit harness, start by laying out the components as to where they go, and start plugging things in. take your time.
if this is a universal harness, the start by locating where you want the fuse box to be. next group all the wires as to where they go, dash wiring, tail lights, engine harness, etc. the wires are clearly marked. again take your time with this. you then need to route the wires where they need to go, neatness counts. once you have the wires run, you can then trim them to length and terminate them.
trust me. if your running a painless universal. it's not so cut and dry as terminating what wire goes were. If it's an direct fit. painless has a vid on you tube to give you a hand. there website has all of the manuals on there but of course you will get one with the kit.