If you've accessed the Generator, you're probably already familiar with the timeline feature found in the single Banner Editor so using the one from the Banner Generator should be a breeze.
The concept remains the same with a few additional features like pasting an animation on all of the other sizes. You can read more about this in our related article regarding middle animations, here.
Using the timeline in the Generator
Show or hide the timeline using the "Show/Hide Timeline" button in the upper right corner, change the duration of a slide, set the build-in/build-out animation duration and delay, customize your animations using the advanced mode, rearrange layers and much more; but, if you're new to this, you can check this guide for more details.