• go to any nostr client or whatever way to generate a nsec.
• do not use your regular nostr posting key, just generate a dedicated one for logins. Is better to keep them as separate keys.
• you can use that nsec into Alby extension (import it) or just use alby extension with whatever new nsec random generated. In Alby also you can have multiple accounts with multiple nostr keys
• you can use that nsec into no2x extension or any other nostr signer you like. On nos2x firefox extension you can use also multiple nostr keys/identities.

Yes you can use coinos to login with LN, but I would use it as backup for nostr login.
Always have a backup login.