With right sides together, match bottom seam to side seam. Pin and sew. Repeat for other corner of bag.
Sew the bottom of the lining in the same way. Trim seam to ¼”.
Turn the outside of the bag right side out.
Insert outside of bag into the lining.
Pin and sew around the top of the bag.
Gently pull the outside of the bag through the opening in the side of the lining.
Push the lining down into the bag.
Press the top edge. Topstitch close to the top of the bag.