For your base concrete  follow our recommendation

1. Excavate your site a depth of 6"minimum.

2.  Fill your hole with gravel 3/4".

3.  Make the form for your base of the exactly size of your model.

4. That your base is levels.

5. That your base is square.

6. Do not pass of trowel on the outline.

7. It is very important that the base is 4" higher than the grass.