This is also another easy part assuming you have dig the hole for the pond something like this:
[attachmentid=3597363]

The base has to be at least 6 inches thick. 8 inches is ok, anything thicker than that is a waste of concrete and effort. If you need to run bottom drains along the base from one end to another you either dig drains and lay the pipes on in it or you can just leave it on the ground but make the base thicker at about 8 inches.

You need some bricks, rebar (reinforced steel bars), rebar mesh, rebar ties (thin steel wire to tie the rebar together), pliers for tightening the rebar with wire.
[attachmentid=3597395] [attachmentid=3597396] [attachmentid=3597397]

[attachmentid=3597399] [attachmentid=3597400]

1st, level the ground
2nd, optionally, compact the ground/soil using a brick by lifting the brick and throw flat on the ground one area at a time. Do not forget to wear gloves.
3. spread bricks all over the the bare base 2 to 3 feet apart. This is to make sure the rebar or rebar mesh not touching the soil and sit on the bricks.
4. Cut to size the rebar mesh cover the whole base to be concreted. Lay the mesh on the bricks. Tie the rebar meshes together with steel wire using the pliers making 2 turns is sufficient and try not moving them unnecessary. Make very sure the rebar or rebar mesh not touching the side soil walls and soil ground. (All rebars must be concealed in the concrete not exposed to air or soil. When rebars are exposed they get rusted and rod away into inside of the concrete and the concrete becomes weak).
5. Cut rebars to 4 to 5 feet longs and bend them into "L" shape. Tie the bottom side of the "L" to the rebar mesh and the other standing along all the edges (walls). Then using other rebars tie all the "L"s together so that is stand straight. These rebars are for the side walls connecting the base after the base have been concreted poking out along the sides of the base.

Now, you are ready to pour the concrete mixed.

As you can see, the brick is 3 inches thick on its flat side and when your concrete is 6 inches thick, the rebar mesh will be at the middle of the concrete.