I used a base by =Kencho
and a drawing program. For this one I used paint because I used to fail at photoshop, but now I just use photoshop.
If you have photoshop, open up the stamp template and make a new layer below the template. Then, paste in the image you want and resize as needed. Finally, erase the excess by either selecting the outside of the stamp and switching to the image layer and pressing delete, or by selecting the inside of the stamp (use the template layer for this), switching to the image layer, copying that, deleting the image layer, then re-pasting the copied bit into the selection. I-if that made sense. (don't forget to flatten the image when you save it lol)
It's a lot harder to do in paint, though D: You'll need to get the image you want all sized beforehand, then put the template on top, then erase the excess.
I hope that helped? There might be a tutorial floating around too