Congrats on the appointment!
Best advice is keep it simple, concise and coherent.
Break it down into segments
Pre match
Confirm who is senior and junior and what you want from each of them (full match record for senior and eyes and ears or goals from junior).
Talk to senior on how you want subs and benches managed.
Kit checks, tell them what you want re sock tape, undershirts etc.
Handshake- break check nets and come back in to witness toss.
In game:
Cover offside
Cover throw ins and areas of credibility and what to do if you cross
Free kicks and your tolerance levels ref little fouls that you may or may not choose to give
Penalty situations - (usually I'm giving it, I'm not giving it or I'm looking at you for help)
What you want if they observe misconduct player number, what they did/said and what you would recommend as a sanction
Mass confrontation