I want to make a gravestone
Real gravestones are basically carved out of a block of granite. Tools to carve the stone are specific to the craft. Do you want something decorative like for Halloween? Those can be made out of insulation foam sheets. Some are made out of cardboard.
You can make one out of concrete
We made a grave stone for our cat by using a large slate and painting or stenciling the writing on it. It's been in the garden for years and is still good.
I would use a square form made of wood and lined. Cement comes in smaller bags now. Then be ready as it drys to put in name, dates and whatever you like. When it is all very dry I would use a subtle color paint over the name etc. I would then cover that area and spray another subtle color. You could TRY this. I never have.
Hmm still thinking