Let's say the flashback flashes in the protagonist's mind, what can you do besides graying the gutters, or graying the panels, or making the borders fuzzy? Is there a way to make it clear where the end and beginning is, and convey that the thoughts are violently flashing inside the mind of the protagonist? I couldn't find anything beside what's explained in the video.
It depends on the length. If it is an extremely long flashback, then you would want to use a new chapter that begins with italics. For example:
Fifteen years ago:
I stood above the bodies of my parents, the room filled with a red haze.
Another way, if the flashback is short is to just use italics in a new paragraph. For example:
I looked around the apartment and saw that is was similar to a place that I had lived before.
"Jerry! Jerry!" My mom called out as she came out of the kitchen from our small one bedroom apartment