Steps are the same in every app — labels and button names vary slightly.
Open whichever app you installed and start the Sign up / Create Account flow.
When asked which server to use (the field may be labeled Server, Homeserver, or Provider; some apps require tapping Edit or Change first), enter:
https://matrix.markbiswas.com
Pick a username and password. Your username is permanent — you can change your display name later, but not the underlying handle.
When prompted for a Registration Token, paste the one I sent you.
You're in.
No registration token? Reply to my invite and I'll generate one — they're single-use.