1. I use "Dura Weave®" bags. Purina and Walmart/Sams's pet food uses this for their larger packaging.
3. Turn the bag tube inside out. This requires a bit of tugging, but keep at it. It was at this point that I used cleaning wipes to wipe down the inside of the bag to get rid of any food residue.
4. Flatten out the bag and 'square up' the open ends so they're perpendicular to the folded edge.
5. From the bottom (if you wish, mark the top and bottom of your bag to ensure proper orientation throughout the process) of the bag, cut one 'loop' 4" wide. This will make your handles. Don't worry if there are holes along the edge left by the pull string. They will be folded into the handle on assembly.
6. Determine how tall you'd like your bag. I generally cut another 3-4 inches off the 'top,' leaving the tube about 17-18 inches tall.
9. Sew a seam .50" from the bottom edge. I have used nylon filament thread for the project. It's see-through so I don't have to worry about matching colors. It's also a bit stronger than poly or cotton thread. You may also use a heavy duty conventional thread. I use a medium stitch length of 2.8mm or 9 stitches/inch.
10. Flatten corners into 45-degree triangles. Pin down the triangle so the pin aligns the seam and the side fold crease. Mark 2.50" from top of triangle across triangle. (I made a template for this because I was sewing many bags.) Stitch across marking. This determines the depth of your bag—in this instance, about 5".
12. Using your fingers or burnisher/bone folder, lay on a flat surface, sharpen the creases of the previously scored lines along the bottom and sides of bag. If you wish, you may also fold down the top 1.25" edge, but you will need to flatten this out to attach handles.
14. Take folded handle strip and top stitch .25-.375" down center folded edge, then across, turn and down double-fold edge. I do this top stitching on whichever side I like better. This 'prettier' side is the one I'll attach on the front of the bag.
15. Cut the completed handle strip into two 17" long pieces.
17. Fold down top 1.25" edge to inside of bag. Starting on a 'side' panel, top stitch .25-.375" from folded edge all around the bag. Once you get back to where you started stitching, make a mark 1" from folded edge and stitch diagonally to that point then top stitch all around again.
18. Resharpen all your creases and you're done! If you wish, you can wash out your bag again with a little dishwashing detergent, like Dawn. Rinse and hang upside to dry.
Hope this is helpful. I've made a couple dozen lately but this is the first time I've really thought about it step-by-step. If you have any questions, just ask!