First, determine an output size. Is this bigger or smaller than your input size? It should be at least one step smaller for the best result with this one, but it can be of equal size.
IF output is smaller and sequence is set to output size, your video will be bigger than your desired frame. You can have it zoom out to the proper size at the first frame and hold that until you need to move it again. At this point, the process becomes very similar across situations.
If not smaller but equal for output size, you’re looking at doing some cropping, zooming and positioning in your effect controls. Similar for larger output as well, but you’ll have to zoom it where you want.
One standard effect is the top\bottom split of video games. You resize your clips to half the frame, and position one at the top, one at the bottom, over the course of a second or two using keyframes in your effect controls.
Another standard effect is to zoom each to your wanted square view, crop it to a square about half the frame in size, and place one to the right and one to the left. Doing this, you either want to find an area where the color lines up just right, so the split is almost seamless, or you want to place a small gap of a neutral variant color (white, black, or a neon halfway between two of the colors at the edge) that will define the split in the screens, while making sure main subjects are zoomed almost equally. To get black just leave space in between, but for white or neon, you may have to apply another effect or clipping that places a bar where the split is. I suggest using illustrator to make a PNG with an alpha channel and then lay the png on a video track over the split screen area and allow the other videos to show through.