As long as you are a member of the Chain Gang group on Bkool.com, the league races will automatically be scheduled.
If you want to ride (or re-ride) a previous league ride, just click on the ride details for one of us who has already done it, and press Challenge. You can then add ghosts/bots if want to under Sessions->Scheduled. When saved (EDIT button) you must afterwards select it from the Leagues section in BSIM and do the ride.
But REMEMBER you HAVE to do the ride before the current league closes, and preferably the day before! This months league closes today (27th), so even if you do one today, you risk it not being approved and it will not count.
If you do it early in the day on the last day AND ask one of the group/league admins to approve it ASAP, then MAYBE it will count. But there is no way to be sure, that the admins read (and act) on your request, and if they do, that the Bkool system includes in it the final ranking when it processes it in the evening. I think the time of day is included in the closing time, not just the date.
Joining the live races is the better way to ensure your rides count - and more fun for everybody!
The league admin has to approve each ride in the league. They can see some basic data, and if things look fishy they can choose NOT to approve it. That could be like a 20kg weight loss (or gain) since the last league ride, indicating someone was a bit creative relative to the route profile.