Most yard haunts or spooky horror flicks are not complete without a few fake tombstones. So here is a few places to go to to find out how to make your own tombstones:
Keeba's Tombstones - by far one of the best tutorials on the subject.
Build a Beloved Tombstone
Build a Grim Reaper Stature/Tombstone
How to use spray paint and masking tape to etch a fake tombstone
A cool tutorial over at Stolloween on creating character tombstones