This part is the tricky one. Use the help of pins.
On one side of the base, the shorter sides, I left it open to insert a piece of cardboard to give strength to the base and remove it when I want to wash the bag.
With the base pieces already cut, I took the body of the diaper bag to sew them together.
Make a hem on one short side of the lining fabric piece.
Center raw edges of both ends to the base right sides together and pin in place.
The body would be sandwiched between both fabrics of the base.
It should look like a sandwich: fabric base-body-lining fabric base.
Pin in place around the edge of the base. The best way is to let the body corner seams must coincide with the corners of the base.
Make sure that the opening of the base (lining fabric) goes inside the bag with the lining fabric. These two pieces should be right sides together.
Start sewing from the middle of the long side of the base where your diaper stacker opening is.
As you stitch through the corner, move your body fabric away so you don’t accidentally stitch it.
When you hit the side that is supposed to have the cardboard opening, sew only the body with the main fabric and leave the lining fabric out.
The whole fabric from the body will be coming out from the opened side of the base.
Right there you should be able to turn it inside out.
Then I cut a rectangle of cardboard to the same side of the base and I wrapped it in contact paper so it would last longer.
I inserted it through the opening on the base.
And I made some invisible stitches to secure the opening. But this is totally optional since you might want to remove it to wash the diaper stacker.
The body and base are finished.