(one part of) an advanced flow needs to have a start button.
You can add it being the only trigger, or in parallel to other trigger cards:
That’s the only way to manually start advanced flows AFAIK.
.
First you have to trigger a (part of a) flow with one of these WHEN cards:
and make sure to save that flow before continuing.
Now you can select that flow from another flow when you use one of these THEN cards:
.
More Advanced flow ins & outs:
