Guilds such as this, as far as I know, were recent inspired by the Spotify model. While there are other team structures in that model, Guild is interesting because of the focus is on learning.
A guild could be organized around any topic, but a good place to start is often around development practices for example or testing or anything else that the organization does a lot of and wants to collaborate on how to do much better. If the learning slows down or attendance drops off, then simply disband the guild and move on.
Often you need a champion to declare the need to form such a guild. This champion might be an expert in her own right but wants to drive consistency and improvement across that org or discipline. This champion should get formal approval from leadership as there will be some expectations around participation and potentially commitments. Once approved then broadcast to the potential participants to attract people to the new guild with a kick-off to set the charter and such.
A guild should meet on a regular basis like once a month at least, but since there might need to be guild work to accomplish there might be time needed between meetings to do or prepare.
To get a guild started you merely need a champion to help organize it then a passionate group of people interested in the topic to absorb or contribute to for the good of all.