This functionality is available via the Custom Games button on the main menu and this button will be activated for a thousand users at a time in a gradual rollout over the next 24 hours. As we discussed in Mark's Dev Jam earlier this week, this gradual rollout means that we won't have to take down servers if Custom Games causes stability issues for the rest of the game.
If all goes well, we hope to have Custom Games Mk.1 rolled out to everyone within the coming days. If you don't happen to be in the first 10% of players who are randomly selected to be part of the launch, please be patient; we're doing all we can to get this launch right for everyone.
Our Community Manager LeftofNever has put together a handy video which goes through all the details on what functionality is launching with Custom Games Mk 1. Alpha.
https://www.youtube.com/watch?v=OKqVuaYn7qc
It's worth noting that even if you aren't part of the early rollout, you will can still join Custom Games if invited by the host. If we do detect any issues with Custom Games Mk. 1 over the coming days, we will temporarily disable the functionality while we investigate the issue.
Please keep an eye on this blog post for updates on the rollout process
UPDATES
UPDATE #1 - 12:56 (UTC+1): Custom Games MK.1 Alpha Now Live to 10% players who are active on live
UPDATE #2 - 14:10 (UTC+1): All has gone well with server stability at 10% being given access to Custom Games Mk.1 Alpha, so we've upped it to 50%.
UPDATE #3 - 15:30 (UTC+1): All has gone swimmingly well with server stability at 50% of players being given access to Custom Games Mk.1 Alpha, so we've opened the floodgates and given everyone access to Custom Games Mk.1 Alpha! Take a deep breath, it could get a bit rough for a while. #dangerzone