Designing your own automation workflow is very easy! The first thing you need to do is choose the right type of trigger among the available trigger types. The "When someone subscribes to a specific list" trigger type will fire up your automation when someone subscribes to a particular mailing list from the ones you've created.
All that this trigger requires is that you define the name of the list which your subscribers need to subscribe to, in order for your automation to get triggered! You can find the available mailing lists on the drop-down menu; keep in mind that only subscriptions to this specific list will trigger the workflow - the rest will be ignored.
After you're done, click on the Save button to have your trigger ready and waiting inside your workflow.
This is the first step on creating an automation which will be triggered whenever someone subscribes to a specific list! Learning how the available Conditional / Control steps and the available Actions work, will help you finalize your design.