From the game’s main menu, you can find the custom match option in the bottom right where “Change Mode” can be seen. When you then choose to ‘Start’, all players will be sent to the custom match lobby.
The Party Leader can then customize the game’s map, game rules and the level of any bots that may be in the lobby. Other people in the party will see these rules on their screens. The party can go through picking their loadouts and then ready up as normal whilst these rules are being set. If a player's loadout doesn't match the game's rules, it will be changed, but you'll still be able to continue to make changes until you've readied up.
When everyone is ready – your game will start as normal.