Find a colleague with whom you would like to work in this way.
It's important that you both have as least some experience of coaching, and that you can agree formal boundaries to what you are able to discuss together. For example, it's easiest if you don't work directly with each other or have close colleagues in common. One person may well want to discuss challenges they are experiencing working with a colleague, and it's much more appropriate to have this conversation if the person who is acting as their coach does not have a close relationship with the person they want to discuss. You will find other guidelins to this coaching conversation in the co-coaching topic briefing.
Find an appropriate location and time slot.
The first half of the time is spent with A as the coach and B as the coachee, and the second with the roles reversed.
If you find this way of working together helpful you can arrange follow-on sessions on a regular, or an 'as-and-when' basis. If you choose the latter, it's simply a matter of picking up the phone, saying "I have somethingI'd like to think through - can you give me half-an-hour of your time?" and take it from there. You don't both need to have something to discuss each time. Maybe one time A would like some help, and the next time B and so on. If you build up a small network with like-minded coaches you may decide to get in touch with different people at different times depending on the topic, availablity and so on. This might even grow eventually to a network that has occasional meetings, asks people to come and speak, and so on.
Good luck!