What must we tell the referee about our front-row players before kick-off?

Law 3.10: before the match, each team must tell the appropriate match official:

• who its front-row players and possible front-row replacements are
• which front-row position(s) each can play

Only those players may play in the front row when scrums are contested, and only in their nominated positions.

Law 6.4: before the match the match officials inspect players' clothing and studs for compliance with Law 4, and check the availability and status of front-row players.

Plan for this: in a squad of 16 to 22 players you need at least four or five front-rowers, and your bench must be able to replace a prop and/or hooker at the first time of asking (Law 3.8).