I will try to explain as good as possible but I know that I don't usually express myself very well. I hope that the photographs are good enough and they can explain by themselves in case I didn't to it in my writing.
This is the paper I used. I was a big scrap paper I had left for another project. I don't know what is the size I used here but it doesn't matter for this tutorial. I like making bags from patterned paper because the result is beautiful and they make a great addition to packaging.
1. Take your paper and fold one of the sides to the half of the paper. Then fold the other side, overlapping of at least 0.5 inches the first half folded. Make sure that the lines are perpendicular to the top of the bag.
2. Glue the sides or add sticky tape so you have the first part of the bag. I realized that my paper wasn't well cut at the bottom after gluing it, but it doesn't matter because when I finish bag that is not going to be seen.
3. Now, for the depth of the bag I folded the bottom of the paper to my desired depth.
4. Unfold the bottom part until you see the hole, and press in both sides and make a triangle in each side. You can cut the part where both triangles meet. I didn't do it but it is going to be proably a lot easier to fold if it is cut. Use your fingers or your tool to mark the folds properly.
5. In the upper part, make a fold that overlaps the middle part of the bottom. You can add some glue to attach the sides to the bottom.
6. Do the same to the bottom part and make it overlap so you can glue both parts together to make to bottom of the bag.
7. On both sides from the bottom, fold form the edge to the corner where you folded the upper triangle. Make sure that the line is perpendicular to the top of the bag and mark the folds very well.
8. The bag is already made. Unfold the bag and press the sides you have just made to the inside. I like to press very well everything once I have finished the bag, so it is easier to storage. Now you can add holes, decorate it with tags, etc. If you want to use the bag for something quite heavy, make sure that the bottom is well glued and that, if you make holes, you have so reinforcement for them, like cardboard for example.
I hope you have enjoyed the tutorial. If you don't understand something, just leave me a comment and I will answer as soon as I can.