I had originally seen pumpkins like this at a local decor store. I really wanted to buy some, but the smallest ones were $12 each and I just couldn't swing that. I ended up buying some of the craft pumpkins (I chose the white ones because they're different and i liked them a lot). If the pumpkins are out of your budget, wait until after Halloween and keep and eye out for a good clearance sale. I am totally getting more for next year if I can find them at 75% off!
Now, these aren't prefect straight lines, but that's not what I was going for. I did a lot freehand. I started by measuring around the widest part of the pumpkin, then dividing that measurement up evenly. On the small pumpkins I made a mark every 1.5 inches, but on the large one I made the mark every 4 inches. Just do whatever you want whether you like the smaller diamonds or larger ones.
I didn't draw lines to fill in, I just made little dots and did everything else freehand with the brush and the marker.
These are very easy to make, and they fit into that category of decorations that I can leave out after Halloween because they're great for fall and aren't covered in bats and skeletons :)