I’ve moved this topic to the discussions forum so we can keep the calls for submissions forum for active calls.
In regards to your query, the best advice I can give is to pay attention. Find a call that you like and pay attention to what the person putting up the call is asking for.
The what comes down to finding a call you’d like to submit to. There are calls for all sorts of things.
The how comes down to the call itself. Some people are asking for short stories about a specific subject to a certain word count. Other people have broader specs and fewer details.
If in doubt about any of it, ask. As someone who has put together a few collab zines, I’d always rather be asked in advance.