Okay. We gotta get some sand into a pit on F-2 so we can cross it so we can hit a button on the room that has the stairs to F-3.
First, get yerself to the right side of F-1. Actually no we don't. Just get to the boss key however you'd like, or even anywhere
near the boss key. On the screen to the north of the chest containing that boss key, on the vertical column of blocks on the right side, push the fourth block from the top. It's tan. It can be pushed either left or right. Walk on the warp it reveals. It's actually pretty straightforward from there. You'll kill some enemies and then go left to find a chest in a big ol' empty room with a tan and gray checkerboard pattern.
IMPORTANT: In this big empty room, there are pressure plates that make you "fail" the room. You have to hug the wall counterclockwise from when you enter, then only leave the wall when you're just below the chest. See how some of the tiles have a darker border? Don't touch those.
Anyway, when you get to it, It'll have a blue dot. That's the "trigger" that makes the sand fill the pit. It'll also make the pressure plates descend. You'll see what I mean. That means you've won the game.

Well, the room, at least. Small steps, I guess.