A slightly easier way that I've done is it after your material is washed lay it out folded over with the non-cut sides together, make sure the right sides of the material are together. Fold the material over again so that the first fold is even with the non-cut sides.
On the 2 cut sides, cut them so that each one is a straight line(really easy with a rotary cutter & mat). Take your ruler and measure the long length of the material to find the middle. Cut down the middle to make 2 pieces of fabric.
Set 1 piece aside and follow the rest of this using 1 piece.
Open the 2nd fold you did up so it is only the 2 non-cut edges touching each other. Sew the 2 layers of each cut side together using a flat or french seam. You're going to want at least 2 rows of stitching if you're going to be putting cans and such in it.
On the bottom, open the fold up as wide as you want the bag to be. Sew the triangle across it to make the gusset.
Hem the top edge of the bag.
Add the handles, if you're putting cans in you're going to want a stiff material like canvas. When sewing the handles on, sew a square around the outside of where it attaches and then an X to reinforce it.
If you use the natural fold of the fabric, you will have 1 less seam to have to deal with.
Once you make 1 bag, you could always measure out a piece of cardboard or something the size of the bottom and add that in to reinforce the bottom. Interfacing(for stiffness) and an extra piece of material the size of the bottom would work too.
