First, grab a plastic bag from your store that hasn't been opened yet (makes a nice lazy pattern). Stick this on top of your t-shirt with the neck hole centered in between the bag handles.
Cut around the bag, but do not cut the shoulders area. On the right and left side I left a 3" strip for the "gussets."
Flip the shirt bag inside out. Do not sew where the arm holes would be. So just sew around the bottom of the bag starting from the bottom of the neckline on one side to the other.
Now you flip it inside out and viola! An easy handmade reusable grocery bag, but they are a bit inconvenient for the store clerks if you just hand them to them as a wad, so this is where my twist comes in. Cut off two arm sleeve hems and cut it so it makes a long strip.
Then cut about a one inch hole in the handles of the t-shirt grocery sacks. (The picture shows the holes to the inside since that is where the plastic sack holes were located, but I have decided that straight in the middle would work just as well and be stronger.) Thread the arm sleeve hem strips through the pile of bags through these holes and make a simple tie to keep them all together.
Then when you go to the store, you can take your whole pile of bags and slip the bags onto the store's plastic bag holders with one easy step. Then, untie the arm sleeve strips once they are in place and throw them into the bottom of a bag. Now they are ready to pack and pull off individually.
I made a test run of these to see how it would work. I ended up having to show the elderly lady what I meant by placing them on the hooks and taking off the strips. I told her to load them up heavy to test them out, and she put two really huge bags of frozen veggies in one and protested the whole time that it would get them wet, I had to assure her that I could easily dry them. Then she put produce in the other; that didn't really test its mettle, but the frozen veggies did, and the bag worked just fine. It did get a bit damp on the ride home, so I just let it hang from the kitchen drawer pull until it dried. I now feel good enough to make many more in order to have two bunches to give the clerk two bags to work at packing appropriately at the same time.