Pin and sew
Fit the lining bag into the outside bag RST. Line up side seams so they match. I usually put the pocket on the same side as the loop. You'll have to work to get the strap to sit nicely between the two pieces, and make sure the loop is also in between and not sticking out. Pin around the top and sew 1/2"-3/4" down from the edge, leaving an opening about 8" wide for turning the bag. Don't worry if the lining bag seems to stick out above the outside bag. Just trim this.
