Welcome to XSive fun............
need a manual.......... Can be bought or they are available online.....post links if required
First off the bike needs to be ID'd.......... they were made from 70-83 and about 19 different models that had major changes, 2 to the frame and 2 to the ignition system and 6 different carb setups and within those 6, 2 types of CV carbs and lots of other things.......One real good thing is from 74-83 the motor doesn't change..........Confused...........only untill things get ID'd properly.........
Assuming you dont know electrics........and some of these links won't be found in the Tech menu
If the wiring loom is good then that is where to start.......all the loom connections, (couplings and bullet), need to be checked, cleaned/replaced, (if required), again, switches, checked for damage and cleaned, and testing for resistance/continuity, (same thing measuring if power can run through the system without encountering any broken wires,oxidation or dirty connection in switches or relays). Doing this first will make any trouble shooting of the charging system, ignition, lights, or even carbs easier because you know that your loom is carrying the current required so any other discoveries where there is voltage drop can be isolated with confidence.
These links help to understand the electrical system, how it works and some diagnostic...........sort of in order from a starting point.
http://www.dansmc.com/electricaltesting.htm
Very good explanation from XSleo
http://www.xs650.com/threads/what-happens-when-you-turn-your-key-on.9625/
Multi meter it uses, and a How to on Continuity/Resistance testing
http://www.xs650.com/threads/how-to-use-multimeter.27826/
http://www.xs650.com/threads/continuity-testing-made-easy.49917/
Contact of Grounds or Earth......How important....Very
http://www.xs650.com/threads/grounds-so-important.41775/
Pics of the bike...........Very important and most of us cant read and need pics..........does have advantages as these bike have often had parts swapped from other years and the eagle eyes on here will be able to identify any of these...........
ID, vin no's especially and pics........again we cant read........
NOTE: Do not buy anything before asking around on here..................Advice from friends, although well intention-ed, will 99.9 times out of 100 be wrong when it come down to the XS650......
And another thing........if one thread is used for any questions, (basicly a build thread), it is easier for the regulars to keep track, to know the bikes details, and what advice they have given before.......