Determine a size. Then you put stakes in the ground, and make 4 corners to square it off. When you have 4 corners with the stakes. you get your string or rope and you make an "X" and make sure each corner is the same size. Then you get your line level and make sure each side is leveled.
When all 4 sides is leveled you get your plank wood and you dig deep into the ground until you see the tip od the wood sticking out on top. You line the wood to the bottom of the string, you make your box and fill it with dirt or fill which ever you have until it's to the top of the wood. When it is leveled put 1 2X6 piece of wood across the center. Then the rest of the 2X6 of wood you screw each one going across. Do not use a hammer or nails. Hammering will make the wood go down deeper into the dirt.
When you have all the wood screwed in all across the planks. You can leave it the way it is or put carpet grass to last and not get dirty. Now that you have your floor you are now ready to put up your shed.