To make your mold you need a waterproof plywood with a smooth surface. Shuttering plywood is a material that's designed to make molds. It's got a very smooth surface and is water resistant. It's also very expensive. When you are planning on reusing the mold I will recommend shuttering plywood. Otherwise melamine board is a cheap alternative.
Melamine board got a smooth surface but it isn't water resistant at the sides. I found an old closet in the dumpster and used these boards to construct my mold.
I used 10 premixed concrete bags (25kg each). The premixed bags are easy to use. When you want to make your own concrete you have to put the cement, sand and gravel together. 1 part cement, 2 parts sand and 3 parts gravel. When you change the size of the bench you have to calculate the amount of concrete you need. A 25kg bag of concrete is good for 12 liters.
- Silicon sealant
- Release agent: silicone spray, petroleum jelly, WD40, etc.
- Masking tape
- Some pieces of scrap wood
- Concrete coloring (optional)
- Power drill
- Chaulking gun
- Old orbital sander
- Mixing bucket