Barb, a footprint is exactly that - its what you leave behind you at a particular point in time and space.
You can try to describe what's in your mind as it happens, but that's really difficult. I would advise a little room for retrospection - the amount is up to you.
Your next choice is what particular point (or points) in time you select - we advise creating a 'design' footprint (the footprint you as designer are hoping will describe the experience of most participants at the start, or the footprint you as a learner experience/d at the start of the learning event).
You can then add a number of footprints of later stages of your experience of the event, or of most people's experience of the event (if you are a researchers, and are trying to interpret data from participants and visualise it). How many? Depends on how many distinct phases or stages you describe.
Painting a picture is, in our experience as describers of our own experience, a strange experience in itself, and most often surfaces things and thoughts and feelings that you (as footprint creator) were not that aware of. So, who knows what will happen? You have to try it and see ...
The analysis will follow? Its an interesting dream, and a demagogical conceit of Latour's. For us we just want to create a number of descriptions and visualizations that people can use to start conversations with themselves and with others about what 'actually' happended. We can take it quite a bit futher than that, but that's another webinar! We need to cross this bridge first!