Hi! I make new tombstones out of cardboard boxes every year.They don't look as great as the foam tombstones I've seen other HF members create, but they're cheap and fairly easy. They can also be very effective! I'm sorry that I don't have any photos but I'll share my steps with you.
1) Choose a box. More rectangular ones tend to be the best. I usually just keep them their normal box shape, because trying to cut and fold them into a typical tombstone shape takes too long for me. Using the whole box also makes it more realistic than just cutting out the shape on a flat piece of cardboard (although you can do that if you prefer).
2) Tape the box closed with duct tape or packing tape. Secure all the corners and seams to keep out the elements. I always leave one of the flaps free, this is how I secure the tombstone to the ground.Or you could place a brick or something heavy inside before you tape it to weigh it down so it won't blow away..
3) Paint the box. I've used both grey spray paint and acrylic paint. Once you get your grey coat done, go back with black paint and add it around the corners, along the sides, and 'peppered' all over. If using a brush, try dry brushing. It adds a more realistic stone-like look.
4) Add details. Paint names, dates or epitaphs. I always go for something humorous or punny, like "Al. B. Bach".(I love to hear people chuckling when they read the tombstones). You can paint cracks, skulls, angels, demons. Your imagination is the key!
5) Coat the box with some kind of clear coat spray. It's not perfect, but it will help protect it against rain. If there's a lot of bad weather I can't promise they'll hold up, but mine have withstood a couple rainy days without collapsing or having the paint run.
I hope this is helpful! (Also my first post on the forum!)
And another great resource from YouTube will be just about anything Unhinged Productions puts out, but specifically for what you asked, he has a tutorial for Tombstones from boxes. It's multi-parted, but it starts out here: https://youtu.be/mKY85ImmfKo?t=30
Hope that helps you. Make sure you get them good and watertight.