I've seen a lot of different video on how to make cracks. I don't subscribe to the neighbor kids stealing your tombstones as you may not get them back.
I've used acetone, that works but does release a toxic gas. But it does work. To use acetone you need to know where you want your cracks and to start something in the area or draw an out line. Don't worry whatever you draw isn't going to look the same when you're done but it does give you a good idea where you want to direct your spray. Get an empty spray bottle and mix some acetone with water not much water but some to dilute it a bit. You can do it straight that works but be very careful.
You can also take a hot gun and just carve out some cracks. Oh btw this is for form only, these crack techniques won't work on wood or real cement.
You can also use a knife I saw a video of a gun using a knife. Took some time but it worked pretty good. The amount you want to cut out will also depend on the tool you use. I've seen a wire brush lightly used or roughly depending on the look. I also use a stiff nylon brush but that is to smooth out areas of hanging foam.
I use a large 6 to 8 inch deck screw. It works great and doesn't take that long. Just take the head put it in your hand and use about an inch or so of the point to scrape across the area you want to distress and crack. It works great.