Simplest explanation
1.) Visit https://nostr-broadcast.pages.dev/
2.) If you have a NIP-07 extension you can click 'get from extension' otherwise paste your PUBLIC key (npub) into the box.
3.) Click 'Fetch and broadcast' and wait while it finds your notes and broadcasts them.
Advanced
If you want to use your own list of relays:
1.) Save site or clone repo
2.) Edit js/relays.js
3.) Open index.html with a browser and follow the steps above. It will now use your defined list of relays you want things to go to.