It looks like you are trying to sew both sides the binding on the machine. As you will see in Heather Bailey's tutorial, the second side is handstitched down. I've tried many methods to sew the second side of the binding on the machine but it's almost impossible.
I do my binding just like Heather Bailey - cut strips 2.5" wide, press in half lenthwise then match raw edges of binding to raw edges of the quilt and sew around - mitering the corners as you go. Turn the folded edge of the binding towards the back and HANDSTITCH in place. This method is neat and doesn't show stitching on the front of the quilt. It's up to you whether you use bias binding or self-made straight of grain binding.
For those that are interested, here's a description of the interfacing I used:
Pelmet Vilene (New Zealand) Extremely heavyweight interfacing used to make valances for window treatments. It is very sturdy and will not wrinkle when sewn over repeatedly in the same area. Timtex (US) used for making fabric bowls is the same thickness, as is Pellon #70 interfacing. As a thickness comparison, take a piece of paper you use in your printer, fold in half and half again and half again. This will give you 8 small divisions. Run your thumb and fingernails along the creases to make them really sharp. The thickness of this fold should be the same as the interfacing.
I want to add a gusseted zipper to a square bag. Is that possible? I'm making something similar to the "Pleatness Bag" and would like for that zipper to not be at the top of the bag so that I can do a fun little tab with a brooch on it.
Looking at my personal store bought bag that is similar, I see that they don't attach the gusset at the ends of the bag. I could do that, but I'm wondering if I did something similar to your tabs at the ends of the zipper, only make the entire width of the gussets and the zipper have a tab and sew that to the end of the lining...if that'd work.
Thanks in advance for any help you can give me.
If you did a square version, the zipper would sit too far down in the bag to make it practical.
To make the square bag possible, you would make the gusset like you said with your store bought bag - ie. it would be open at either end.
It was time I made my other son some luggage too. So, here it is!!:
Yes, it's for a boy - an 8 year old boy!! He chose the fabric including a hot pink plain fabric for the lining!! The outer fabric is called 'Bad to the Bone' by Robert Kaufman.
I used a super heavy duty interfacing. I literally wrestled with it on the sewing machine but I'm extremely pleased with the result. The bag is empty in the picture so it is standing up by itself. It's almost like cardboard. I would highly recommend it.