Turn the lining to the right side and leave the exterior wrong side out and insert the lining into the exterior fabric, making sure the side seams are lined up.
Then, grab whatever you've decided to use as a closure, and create a loop (as shown in picture 3) that's big enough for your button to go through.
Place the closure on the bottom of the bag, as close to the middle as you can get it (measure if you like - or just eyeball) and pin in place as shown in pictures 4 and 5.
Now, pin around the top of the bag, placing the pins horizontally to the top, except for the side opposite the closure, where you'll place two pins vertically a couple of inches apart. These pins will be your markers to start and stop sewing, leaving you a hole to turn the bag right side out. (See picture 6)
Sew 1/4 inch in from the edge using a backstitch
all the way around except for the place you've marked as a opening, knotting well at the beginning and and the end.