The first thing you should keep in mind is that there is a limit to how many custom matches can occur at a particular time. It depends on various factors, such as your Cloud PUBG server. To give you an idea, PUBG allows approximately 10,000 custom matches to take place simultaneously to maintain system stability.

• Download the PUBG test server from here
• Install the test PUBG server and launch PUBG
• Go to the Play tab and choose custom match
• Choose one mode from Normal, Zombie, E-sports and War mode
• You will be taken to the next screen, where you will see all the active sessions.
• Select any session to join the game
• If you don’t want to join any match, you can create your own custom match by clicking the create button present in the bottom left corner
• You will be taken to a new window where you can choose other options and presets.
• Once done, press the create button
• The administrator can set up passwords for private events as well as spectator passwords

If you need more information, you can check 

PUBG guides

 for more information.