必ず本人が登録用紙に記入して下さい。

。

The person themselves must fill out the registration form.

Literal

Certainly person-themselves [subject-が] registration form [to-に] fill-in [polite-request-ください].

必ず本人が — 本人 means 'the person themselves' (not a proxy). 登録用紙に記入する means 'to fill out a registration form.' This is a firm administrative instruction.