Easy Answer: There are hundreds of ways. Most of them involve sewing.
Reusable bags are big business these days; nearly every store will sell you a bag for a buck or two...or more. But you don't have to pay for a bag that advertises where you shop. You can make your own.
I made the first bag! Are you impressed? I used a plastic grocery store bag as a pattern and worked from this Craftster tutorial.
Bag two is slightly more involved tote pattern posted at SewMamaSew. I haven't tried it out, but it looks pretty straight-forward if you know how to sew.
Number three is from Martha Stewart. The crafting-queen turns old and thrifted t-shirts into shopping bags. The only sewing is a straight seam along the bottom of the bag. ( For those of you who don't want to sew: I wonder if it'd be possible to seal up the bottom of the bag with a bunch of safety pins? Or some fabric-glue?)
And bag four is made from a vintage pillowcase (the tutorial comes from a blog called Creative Kismet). It may be time to dig through grandma's linen closet.
If you've got a favorite bag tutorial--or want to show us the bags you've made--post away!