I recently bought a V-star 650, the dude told me that he replaced the battery with a smaller one and that’s why it had no electrical power. I replaced the battery with the correct size still nothing no horn no lights. I replaced all fuses including the main fuse and still nothing. Please help I’m getting extremely frustrated.
Your question is broad, and difficult to comprehensively answer in this venue. I suggest you find a automotive - electrical text book, and follow the general recommendations in that book.
You will want to verify that the ignition switch works. Any load relays work. The fuse panel is correctly distributing power, etc. You will need a voltmeter, and it will be helpful to have some alligator clip jumper wires. Since your battery is not charging, it may be helpful to have a battery charger available as well.
The debug is rather easy to do, but requires a little more content than can be written in this venue. You may also find "illustrated" instruction in the form of youtube videos.