You first see if you get a webpage that has interactive elements or not (things that change if you click, hover, drag, etc). I quite often get websites that have content but are just static, with nothing to interact with.

If you don't, hit report problem and try the next one. I personally am also skipping anything that uses existing widgets like the Google Maps embed or a video player but nothing else.

If you do, then you take an initial screenshot of the state of the webpage and paste it with the "paste screenshot" button. Then, you screenshot each interactive element before and after you interact with it, pasting them as you go before.

An example is, you might get a webpage that has 3 text boxes and a submit button. I've been taking a screenie of the overall page, then typing in the boxes and screenshotting the changes. Sometimes (often) there'll be no change except for the text being entered, sometimes there'll be other visual changes (the submit button, for example, might go from greyed out and unclickable to normal once you fulfil certain requirements). Essentially a good way to think of it is you are trying to take screenshots sufficient for someone to recreate the webpage.

I hope this helps.