Map the system
Ha! This is a bit of a trick answer ;)
So here's the thing, almost everything we do in systems thinking is a version of 'mapping the system' That's because almost all methods have a basis in visualizing an aspect of the system and showing parts in relation to each other.
Challenge is, there are about a 1000 different ways to do that! And each method only gives you a wee perspective on one aspect of the system. This is also why we need to LAYER multiple methods to see more of the system.
So, from here you might like to 'give something a go' or go back one step and think a bit more about what you're trying to learn about or do in the system right now. Almost all methods will have you arrive at some version of a systems map :)