In this able I'm going to show, how can you create a cheap concrete garden bench.

Step 1: Plan

In these pictures you can see some plans with dimensions, designed with a 3D designer program.

Step 2: Ingredients

Materials you will need:

Some wood boards
concrete (either instant or gravel and cement)
screws
plastic/wood sticks
OSB board
plastic bucket or tube

Step 3: Assembling the Scaffolding

To make the scaffolding I used OSB board as the base and first I drew with pencil the lines.

Then I fixed the woods with screws as you can see in the pictures.

With some plastic sticks you can keep the holes for the screws, so in this way you will not have to make the holes in the concrete with drill, just drill out these plastic stick.

For the big hole you can place a plastic bucket inside the scaffolding, to do that you will need less concrete so the product will be less weight furthermore you can fix the seat wood throughout these hole, as you will see later.

Step 4: Concrete

I mixed manually the concrete from gravel and cement and water, to be strong enough you can use 4:1 ratio (4 parts gravel 1 part cement) and water as needed to be not too liquid.

Step 5: Fill the Scaffolding

I put a stump on the bucket to keep in place during the filling process.

First we need to spread the bottom layer with the half amount of concrete and ram it to let the concrete go into the all of corners. Then we can use any kind of junk metal parts (rusty pegs, screws, rods, wire and so on) to make ferro-concrete, in this way we can multiple the carrying capacity of our bench. After we placed the metals we fill up the scaffolding with remaining concrete. With a masonry spoon we can straighten out the top.

Just a tip:
If you do not want hard edges, you can use for instance polystyrene foam in the corners to the edges be rounded. (as you can see in the last picture)

Step 6: Exposing the Concrete

An important note before you touch the whole stuff: please keep the time the concrete requires to be strong enough. Otherwise you can brake it easily (my own experience :-) ), in general you have to wait at least 3-4 days (the more is the best), unless you use extra rapid concrete.

To make the concrete to be free is a really simple process, just screw off the screws and you can remove the scaffolding.

Then you can use drilling machine to make the holes with removing plastic sticks for the screws in the concrete.
The best is a drill bit for hard materials.

Of course you will need two pieces of this concrete stuff for one bench, so it is a bit longer process.