I make my bottle cap images in my graphics program. If I had not gone to school for art I would have had no idea how to make them either. I don't mind explaining how I make them. I don't know how other people make them but this is how I make mine.
I save the picture I want or I draw a graphic with my mouse in my art program. If I am using a picture I have to re-size it, if the image is square I then Need to make a 1 inch circle that has no fill in the middle but with a 1 point border so I can see where I need to crop it. Then I have to use my pen tool and crop out all the areas that are not within the circle and fill those areas white.
If I draw the image my self I zoom in and draw it large until I like my design. Then I group the image and resize it.
I make a 1 inch circle and fill it in the color of my choice.
Next I grab my image I created, size it and place it on top of the the 1 inch filled circle.
If I had a way to capture each step in a photo I would make the instructions. This is the best way I can explain it, it is time consuming to make the images.
These are some sample images of bottle caps I made by drawing the art with my mouse.
This would be a sample of caps made by using existing pictures and cropping / editing them except for the sweet & sassy which I typed and text the text within the filled circles.