My first suggestion is try to find a used harness from a breaker or someone like Hawk Suzuki ...
Before I go any further, I want to make it very clear that I have hand built a wiring harness before and unless you have some degree of experience with auto electrics, along with a source for the required connectors and wires with the different color insulation, you're going to be creating an even bigger problem than the last guy who worked on it.
Building a wiring harness is not an insurmountable task, but it does require some knowledge of what you're about, and Suzuki uses "negative switched" headlight circuits, which most people find quite mystifying - which may be the reason you are faced with the task you are.
Next get yourself a diagram - either from an FSM or from ack's faq - you're going to need it, whether you build your own harness, or buy one.
If you build your own harness, you'll need to find wire lots of it, in different colors, and since you probably won't be able to get the colors the factory used, you'll have to make your own notes as to which color you used for what connection, and then created a "modified" diagram showing your changes - or you'll have an even bigger mess the first time you need to troubleshoot anything (which will be before you complete the project).
I suggest you work on the basic circuits first - and test each circuit as you complete it.