I dont know how your client is setup, but in Amethyst it's easy to find you list of relays and add new ones. The three that allow cross-posting from Mastodon are:

wss://relay.mostr.pub

wss://relay.shitforce.one

wss://nostr.coinfund.app

Your posts go to the Fediverse showing up as @mostr .pub, their posts come here as _at_@mostr .pub, and you can interact between protocols