Luckily, it is a really easy thing to do. Here is how I do it.
First, I lay my finished purse on two layers of the lining fabric. Using a pen, I draw a line about 1/2" outside the edges of the bag. This does not have to be perfect. No one will open out the lining and say "Oh my! You're lining is crooked!" If they do, slap them. Tell the cops I said it was fine.
Next, with right sides together, sew around the outside edges of the lining. Do not sew across the top.
Tuck the sewn lining into the purse, with the wrong side of the lining facing the inside of the bag.
Turn the top edge in toward the bag and whip stitch the lining to the bag around the top.
For this bag, I started my lining right under the handles. Don't go above the handles or you won't be able to use them. Sounds like common sense, but I've been in a rush and done it. Can't be pretty and smart.