Having a seed in a Coldcard Q, I derived —using a BIP85 index— a deterministic 32-byte-hex. That's a private key for Nostr. 

I asked Q to show it as a QR, and then I scanned it with a Nostr client. Thats it. 

If I lose my phone or logout from client, I can always derive it again using the same seed and index with a CC Q 🙂