Dig it out , even under the frame that's on the ground . As you dig back under the frame you can use bricks to support at each brace and corners as needed. Pour back to level it is now , letting the concrete run under the frame and leaving the supports in. Pour your apron wider than 1' , 3-4 and run it all the way across with a little fall in it. Might need gutters and cut a swell around it to manage water. Almost forgot, anchor the lower frame to the concrete .

Slab that size will need at least one joint sawn in it.