Watch the first three arcs to catch up: Waluigi Bob Anime Then watch the "Meggy's Destiny" movie. From there, watch the YouTube arc in full. Then head to the Glitch channel and watch Sunset Paradise. Then watch "Mario Babies" and watch every recent upload from there.

Or you can go back in his upload history and start at, "If Mario was in Splatoon", and watch every upload after that until you're caught up. I think that's a good starting point if you wanna get caught up on absolutely everything recent.

Whoops wait my bad. After the YouTube arc, watch "Mario vs YouTubers" and watch every recent upload after that. And then there will be a point where SMG4 will literally tell you to watch Sunset Paradise. That's when you watch it, while still watching SMG4.