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.