I think it's great
that you are motivated enough to take on this task yourselves.
You could fix a time where you meet up with your colleague(s). A regular day and time is more motivating and more likely to happen than something vague.
Here's what you could do:
Get some books on teaching techniques and then a) all read the same chapter and discuss it or b) read different chapters which each of you will then present to the others.
Choose a teaching point that you find interesting or useful. You all try it out and come back together to compare how it went.
If possible observe your colleagues, either those in your study group or not. You learn a lot from observing others, whatever level they are at. Both the observer and the "observee" learn a lot and points always come up that can be discussed. Remember to be sensitive during these observations.
Finally, keep in touch with this website