Make it all the color of the top and bottom. Sew your middle color over on the center. Measure where you want it down from the top. Turn and press the middle piece 1/2 inch under on top and bottom and stitch with a double seam like you see on jeans. Then sew up the sides matching the center seams. If you want a flat bottom. Turn the bag inside out, match your side seams with the center of the bottom and pin it across making a triangle, then stitch across from side to side.

The strap can be strapping you buy at a fabric store or made from your material. If you make it from your material, back it with heavy interfacing. Sew it together and turn it out the right way--the rubber end of a pencil can help with this. Keep the seam at one side of the strap and stitch in 1/4 inch on both sides. The straps can start at canter bottom and go all around the bag. Overlap on the bottom by a few inches. Sew the strap on on both sides of the strap. Following the stitching lines already there if you are using material. I suggest you do some measuring to make sure the strap is the right length.

A heavy card or plastic covered in your lining material will keep the bottom flat.

I hope this helps If you have trouble post again and I'll try to help