[Mart]

Decide on the route you want to wires to take.

Then decide where all the junctions are.

Measure the length of the harness between all the junctions and which wires enter and exit at each junction.

Lay it all out like a big road map. Use a big piece of ply and knock nails in at all the junction points.

Lay all the wires in and follow the map so all the wires go where they are needed.

Leave all the ends a bit long so they can be trimmed to length and the right terminals applied.

That's about it.

Hard to describe really without writing a book on the subject.

Draw a diagram. Use an existing simple vehicle as a guide.

Mart