After you've subscribed to each map you want to play on, follow these steps to play with Custom Maps:

1. Launch the game.
2. Press the "Esc" key on your keyboard.
3. Click "Game Options," then click the "Edit" button located to the right of "Maps."
4. Near the bottom-left there should be text saying "CustomOnline." In the maps menu, a white square = on and a gray square = off. Make sure the square next to "CustomOnline" is white, as well as the squares next to every map you subscribed to. (Keep in mind that there will be a second page of maps if you subscribed to more than 8. Click the "RB>" button in the bottom-right to go to the next page of maps if this is the case.)
5. After you've made sure the squares next to "CustomOnline" and every map you've subscribed to are white, exit the menu and jump in the "Host Match" box in the room to your right.
6. Invite any friends that want to join and if desired, turn the randoms lever to "On."
7. Once you kill eachother, your custom maps will now be in the map pool for the lobby. If desired, make the box gray for all of the other categories of maps to disable them and make it custom maps only. Hope this helped.