You need:
• Cardboard
• A pencil
• Duct tape
• Something to measure with. Measuring tape or string will work.
• Heavy-paper scissors - you can get big ones at the dollar store for cheap. Do not use your mother's cloth scissors (you'll ruin them).
• A permanent marker
• Some heavy rocks for the base
• An epitaph

Optional:
• Color markers
• Paint
• A knife if you cannot find scissors - be very careful with this, though. Remember, always cut away from yourself. Where will it go if you miss or after you reach the end of the cardboard? Never cut with someone in front of you. If you're a kid, ask your parents first.

What to do:

Use your pencil to trace the following shape for the front:
and cut it out. And it IS important to trace everything first because cardboard it so annoyingly difficult to cut, it will come out crooked if you do not trace it first.

Now you need a long gray shape for the border. Measure around (see the orange line):
And cut out to that length:
You are going to have to curve this, so I recommend the thinner cardboard (e.g. from cereal boxes and the like).

Now you just need something for the base. Cut out another piece of cardboard (see the green line). Take your rocks and duct tape them to the base (so your memorial won't be blown away by the wind).

Duct tape everything together so it looks like a gravestone. Try to keep the duct tape on the back part.

Write your epitaph with a permanent marker. Decorate the gravestone if you want.

Take it to the place the tree once stood. Good job, you are finished.

PS - Hey, and if you'd be so kind, might you not write "Gravestones For Trees Campaign" on the back? Spread the word!