The key is to isolate individual systems and test them, then components instead of thinking of the wiring loom as a whole.
Where to Start:
First step, Look at your wiring diagram. See how many grounds you should have, and then go through and find them all, look for any others. There is no point trying to track down other issues until you know your grounds are good and clean.
Clean them with scotch-brite or emery paper (300ish grit sandpaper) until the metal gleams on both the connection end and the frame/engine part.
Re-attach with dielectric grease
A lot of problems can be solved with nice clean grounds.
If a connection is super corroded put a new eyelet on it.
Completely replacing the main ground strap would not hurt.
Check the voltage at the battery with the bike off.
Check that your fuses aren't blown.
Check your voltage to the fuse and what it is after the fuse.
Understand your lighting system
I would recommend sketching out something similar to this to simplify the wiring diagram so it is not cluttered with other stuff (lowbrow has a really cool example):
Use a voltmeter to see what tabs on the back of the light are getting juice when it is set to high beam. If the voltage is different on high vs low then the problem may be with the bulb (burnt out filament or power plan (4)). If it does not change, move back to the switch.
I know for my Yamaha Triple the headlight worked like this: One Ground, one hot for low beam and both hot for hi-beam. Once I traded out my bulb to a brighter one I had to change the wiring so that low was hot for hi but only the hot tab was hot for high beam. If the same thing happened for yours you should see if the high beam lights up if you only give the one tab a hot wire. See ill.
See if you can check the wiring in the switch to see if the issue is the switch itself or if it is the wiring between the switch and the lamp.
You can disassemble the switch (it is probably just a knife switch) if that is the problem.
I would normally recommend starting from the battery and tracing the current at each component on its way to the switch but since we know you have juice at the headlight than we don't need to do that.
Make sure you understand where the voltage goes to make your signal work.
Make sure that flasher relay is getting voltage when the signal is turned. If it does not than make sure the switch is getting voltage. There are probably grounds at each of the turn signal stems (where they bolt on) clean and dielectric grease them.
You can jump your bulb to see if the bulb itself works. B/c of the bi-metal switch in the relay if a bulb is burned out it will not have a circuit and the flasher relay will not heat up and therefore will not function.
I had a bad relay on mine and when replaced it worked properly. My flasher relay was one out of a motorcycle graveyard but I have since discovered that you can just pick them up at AutoZone. You can tell which relay is the flasher relay by the wires that go into it (using your handy wiring diagram). You can jump the circuit with the relay out and see if your lights light up (a good test to see if you relay is bad). You could probably just do this step first but it will be good to do the other stuff to get to know where everything is).
How to attach a connection or repair a break.
You can do a better job with a soldering iron but since I do not have one I usually go without. Trim back enough wire insulation to show the copper inside (usually about ½”).
Slip an appropriate length of shrink wrap over wire (make sure it will fit over the connector when crimped).
Cover wire in solder, insert into connector, and crimp. I use hefty channel-locks to make sure it is well crimped.
Slip heat-shrink over connection, and heat with lighter or hair dryer.
For wire to wire connections you can solder them together w/o the butt-connector and then heat-shrink but again, I use butt-connectors b/c I don't have a soldering iron.