Starter relay is easy to wire.
Starter motor cable to one of the larger posts, tighten it up, that is the ONLY wire on that post.
Battery positive cable to the other large post, and all the other wires go to this post, alternator, and power distribution are here.
The smaller "S" post gets the smaller wire that comes from the Key switch via the NSS or clutch switch, this wire has 12volts when key is turned to START, it activates the relay.
If your starter relay has a 4th post, small one, that is the "I"(ignition) post it was used in the past to power the Coil with full battery voltage when starting the engine.
It wasn't used with electronic ignition system for the most part.