Step 1.

Measure a square area in your yard and flatten the ground if needed (some ideas here). The area should be few inches larger than the actual shed dimensions.

Install 9 deck piers - one in every corner, one in the middle of each side and one in the geometrical center of the foundations.

There is no need of concrete in the foundations especially if you have chosen a place with hard ground.

Step 2.

Here is how to install the deck piers: dig a hole which is wider than the deck beam pier.

Fill the bottom few inches with sand and gravel and compact the filling. Put a paver over it to ensure stability

Center the deck pier and level it carefully.

Fill the hole with gravel and ground and make sure the deck pier is stable enough. You need to ensure there will be no water going inside.

Step 3.

Attach three parallel beams over the deck piers as shown on the image.

Use nails or screws to join the parts. Make sure everything is aligned.

Step 4.

Install as many joists as needed perpendicular to the 3 beams.

You may need to use secondary wooden blocks to stabilize the joists.

Make sure there is equal distance between the joists.

Step 5.

Attach two boards at the both sides where are the joist ends. This will fix the joists.

The edges of the boards must be aligned with the ends of the boundary joist.

The entire construction looks as a lattice as seen on the picture.

This forms a very stable construction.

You are almost ready!

Step 6.

Cut a piece of thick plywood with the size of the shed bottom.

If you don't have a large enough piece you can use several small ones.

Step 7.

Here you go!

Use nails and screws to attach the plywood to each of the joists and your shed foundation is ready!