Here is a Flower Box we made from recycled vegetable crates we got from our local grocery. It seems that most stores throw these out, so as the people in the produce department if they have any of the wooden creates, I imagine they will give them to you.
We use landscape fabric, aka weed barrier, to line the interior of the flower box. The liner does not have to be perfect, remember, it will be filled with dirt. You can also use black plastic sheeting or thick plastic garbage bags, be sure to poke holes in the plastic that rests on the bottom of the crate to allow for water drainage.
Step 1: Liner
fold over the excess liner and put in a few staples, trim the liner just below the top wood slat. Fill the flower box with potting soil, not topsoil. The soil needs to have organic material in it.