Bricks, Masonry, Asphalt and Concrete - Pouring a slab for a shed - need advice

I'm trying to get a slab for a new 10'x16' shed formed up. I'm having a problem with the location that I chose to put it however. I'm using 2x6's for forms, and when I lay it out, I've got about 6" drop from the front to rear over the length of 16'.

I tried leveling the lowside and digging in the 16' side, but when I did that, the top of my form was below grade. So now I've leveled the highside and jacked up the forms till it was level, and I've got 6" of air underneath the form board on the lowside.

I've never poured concerte before and would really appricate some advice on how to properly deal with this. Should I fill the form with sand and compact it? I guess I could just pour it as-is, but that would be costly going 12' thick on the lowside.

I'm sure some sort of fill is necessary, but not sure how to proceed at this point.

TIA,
Joe Michel

You should pour it 12" deep on the lowside. If you just backfill the form to a 6" level, that corner will eventually drop, guaranteed.