The easiest way to figure it out would be to clamp or attach the lights where you want them, the switch where you want it (them) and then lay out the wire in neat bundles. Use a fused line between the battery and the switch and make sure all your grounds are good, clean and tight as bad grounds will drive you nuts with wiring. You can use wiring loom to hold the wires in place or maybe small PVC pipe as conduit. I would use soldiered connections and shrink tubing on any connections. You could sit down with your mower and draw on it with colored chalk to get an idea of how the wires need to run. Brush it off when you are done. Couple of helpful tables:
Typical Low Voltage Wire Sizes Guide
Air Conditioning- 10 GAUGE Heater Switch- 10 GAUGE
Alternator- 10 GAUGE Horn Button- 13 GAUGE
Ammeter- 10 GAUGE Horn To Relay- 10 GAUGE
Cigarette Lighter- 14 GAUGE Ignition Switch- 12 GAUGE
Clock- 18 GAUGE Low-Amps Circuits- 18 GAUGE
Coil Wire- 16 GAUGE Parking Lights- 16 GAUGE
Dome Lights- 16 GAUGE Radio To Fuse Blocks- 14 GAUGE
Driving Lights- 14 GAUGE Single Bulbs- 18 GAUGE
Gauges- 14 GAUGE Starter To Relay- 16 GAUGE
Generator (Charging)- 10 GAUGE Stoplights- 16 GAUGE
Generator To Starter- 16 GAUGE Tail Lights- 16 GAUGE
Headlight Circuit- 14 GAUGE Turn Signals- 16 GAUGE
Headlight Switch To Fuse Block- 12 GAUGE CD To Fuse Block- 14 GAUGE
Heater Leads- 16 GAUGE Windshield Wiper & Washer- 14 GAUGE
When circuit is longer than 10 feet, always use the next larger gauge size, which is the next lower even number.
Wire Gauge Selection Table 12 Volt Circuit
AMPS 3' 5' 7' 10' 15' 20' 25'
0 to 5 18 18 18 18 18 18 18
6 18 18 18 18 18 18 18
7 18 18 18 18 18 18 18
8 18 18 18 18 18 16 16
10 18 18 18 18 16 16 16
11 18 18 18 18 16 16 14
12 18 18 18 18 16 16 14
15 18 18 18 18 14 14 12
18 18 18 16 16 14 14 12
20 18 18 16 16 14 12 10
22 18 18 16 16 12 12 10
24 18 18 16 16 12 12 10
30 18 16 16 14 10 10 10
40 18 16 14 12 10 10 8
50 16 14 12 12 10 10 8
100 12 12 10 10 6 6 4
150 10 10 8 8 4 4 2
200 10 8 8 6 4 4 2
Some videos showing what other guys did:
https://www.youtube.com/watch?v=JS8ojXm_Ibs#t=619.291339
https://www.youtube.com/watch?v=5d4cbycv1vM,
https://www.youtube.com/watch?v=azeEZdOm7z0
Tom