The green fabric I used as my ext fabric but the liner is sewn the same way. My ext was 16 x 18 1/2 in my liner was 16 x 17 in.
With right sides facing each other pin the 2 sides & bottom together.
Sew along the sides & bottom using 1/2 in seam allowance. When you start & stop on the other side make sure you backstitch a couple sts.
Make sure you press your seams because it locks your sts into place.
Pin bottom seam of bag to make a boxed corner. Your are good when the pin comes out through the seam on the other side. Take seam ruler & measure 1 in & mark with sewing pen.
Sew along the line & then backstitch over it & sew over it again.
Here is your boxed corner I sew over it 3 times because I need it to have a strong seam. (sew over line, backstitch over what I just sewed, sew over that again)
Cut the material above 1/2 in away from where you just sewed.
In the next pics my exterior fabric is the green & my lining is the pink. It's just when I did this I didn't take pics of sewing the lining because it's dine the same as the exterior.
Repeat on other side. I do this on both the exterior & lining of my bags. If you click on the pic you will see what it looks like.
Put the lining inside the bag with right sides facing each other. Make sure you line the side seams up.
Pin around the top of the bag.
On one side of the bag measure 3 in's & either mark the 2 spots. I mark it with two bigger pins so I know where to start & stop.
Sew around the top backstitching at the beginning & end. Starting at one mark & ending at the other side of your 3 in gap.
Here is your 3 in gap which you are going to pull the lining & exterior of the bag to make it right side out.
I've pulled the bag & lining through the hole & you can see that both are now right side out. Just push the liner into the bag.
Pin at the 3 in gap & then sew a narrow seam all the way around the bag. This will close the gap as well as give the top of the bag a finished look.
Here is my finished bag.