For starters: Use a clearnet lnbits instance, could even be https://legend.lnbits.com . Crete a new account and bookmark the link so you can access it later (there's no other way besides knowing the link). Go to extensions and activate cashu. Then click cashu, add a mint, give it some name and enter the wallet you just create. Hit the green button to open cashu.me. add the mint it's suggesting. Then send some sats there by creating an invoice in cashu.me. you can now mint cashu tokens and send them in Amethyst.