How would I go about making a lining for a bag?

I am I’m the process of making a bag with stripes and I’d like to put a liner in it beause of the changes in color. This will be my first bag with a liner, and I’m not sure how to measure the fabric and whether I should use two pieces sewn together or two sides and the…uh…strip of fabric that goes in square bags…gusset, maybe…Any ideas would be helpful…
Thanks - Elizabeth

EDITED: I would also like to know what would be the best bind off for the top of the bag. It’s knitted in circular with bind offs between the handles. Thanks again!

Here are some instructions:

http://freddyknits.blogspot.com/2005/03/every-bag-has-purple-lining.html

Thanks, Julie! Now all I need help on is what bind off would look the best.
-Elizabeth