Good question!

YOUR dominant hand (the one you write with) is always the one that moves when you sign.

YOUR non-dominant hand (non-writing hand) is the one that stays still while signing.

Sign accordingly!

(Here are some of my Tips for new signers➡️) https://youtu.be/t0i4Eg62XwU?si=Db0Iz3N_KlsrMGHV