Choose a shirt.
You'll need an old t-shirt or tank top, preferably child-sized, although any t-shirt will be fine. Keep in mind that the bigger the shirt you use, the bigger the bag will be. This is a great opportunity to take that much-loved shirt of your child's and transform it into something new.
If using a t-shirt, the first step is to cut off the sleeves. You can use a tank top to trace where the cut should be or simply snip off the sleeves. If using a t-shirt, the arm holes will have a raw edge ~ you could always finish these off by turning them under with a small hem ~ or you can leave them, which is what we did as our bags will be used for beachcombing.

Keep the sleeves for a bonus idea at the end.
Pin at the bottom.

You will need to turn the shirt inside out and line up the side seams so that they are now in the middle (see picture below). This means that the shirt will now have its creases on the front and back. Pin the bottom of the shirt.

Hem the bottom of the bag.

Line up the pressure foot with the hem on the shirt and sew a straight line from side to side, carefully removing pins as you go. As this was my daughter`s first time at the sewing machine, I helped her guide the fabric.
Finished!

Turn the bag right side out and you`re done! Ta da! Now go find some treasure :)
Bonus idea - Don`t throw out the t-shirt sleeves that you cut off in the first step. They make great baby bandanas!