Medium format film is often shot in cameras with a 6x6 square aspect ratio of 1:1 (as opposed to 3:2 in many 35mm cameras). Composing your shots for a square is often quite different to shooting a rectangular image.
Square format can seem a bit daunting if you’ve only ever shot with a rectangular 35mm compostition, especially since the film costs more on a frame to frame basis. The best thing to do is plunge straight in and start shooting. Remember Rule 5. Get closer! and Rule 6. Don't think! It won’t take you too long to get stuck in and find your own unique approach.
With a square format, you can place your subject straight in the center of the frame and create a dynamic composition — which isn’t necessarily true for other formats.
I also seem to get much closer to subjects without the picture appearing strange. Shots that are square are perhaps tougher to compose because there is less area to place the subject on one third of the image (as opposed to 35mm frames).
When shooting rectangular images it is no doubt simpler to compose the shot with a subject using the Rule of Thirds. Likewise, applying a theory like the Golden Ratio can also help produce interesting compositions. While these techniques lend themselves to the dimensions of a 35mm frame, well, I didn’t find them so helpful, initially, in composing for medium format.
Certainly, the Rule of Thirds and Golden Ratio are useful for the subject within the frame but they don’t help me as much when framing up my square shot.
This is where I would be inclined to suggest you fill your square frame as completely as possible with your subject. There is such a large image area that you don’t want to waste any of it by not including information. Where 35mm 3:2 style shots can look good with some space in the frame I much prefer to have the square frame filled up with my subject. Here are some examples: