In order to compete as a master boxer in sanctioned bouts you must have a valid passbook. First, know that Masters Boxing is an amateur division for those 35 and older. If you have competed as a pro or for money such as tough man competitions you are disqualified to compete as a master boxer.
If International, you should use the AIBA Form or have an current book from Canada or Australia.
In order to get your passbook you must have a yearly physical with your physician and have the medical form filled out and cleared as fit to box by your Doctor.
If you are age 45 and older you will also need to have an Exercise ECG/EKG test done every 5 years. Once you have completed your physical and Exercise ECG/EKG for those 45 and older, you need to register on-line with USA Boxing through their website here: Register with USA Boxing
At the end of your registration it will give you the name and contact info for your local boxing commission (LBC) and what paperwork you need to send to them. You will need 2 passport size photos for your passbook. Those can be obtained anywhere including at most local drugstores like CVS, Walgreens or anywhere else that is convenient.
You will then send the appropriate paperwork to your local Boxing Commission and they will send you back your passbook. Bring that and everything else your Boxing Association requires to compete in the tournament.
Once you send in your paperwork, it will take about 3 weeks to get your passbook. Keep your paperwork with you as it will be required when you compete, that is 100% your responsibility. It is relatively simple to get the above however it does take time so do not delay.