First step: make white (or any other color, but I think blacks or whites look nicer) sections for your pictures (yes I'm aware of how uneven these ones are—!)

Step 2: Create a new layer for inserting a photo. I recommend using one for each photo!

Step 3: Insert a photo, and resize if needed to fit into a box. If some still pokes out, use the eraser tool to get rid of it

I think that's pretty much it! Any questions?