If you are having trouble getting panels of any bag to fit together, first make sure your pattern pieces printed correctly. Then, start over and take the three-dimensional factor into account. Some like to lengthen the straight panel (gusset) to be able to fit the raw edges of the curved panel (main panel) and be easier to pin together. This is okay, however the straight panel will end up being a little poofy, as it will gather to match the shorter seam line length of the curved panel. I suggest sewing the pattern as it was designed.
Pin the Gusset to the Main Panel, pinning first at the center. Then pin the ends in place.
Continue to pin along only the straight edges of the Main Panel, there is no fiddling for these points.
At this point, I flip over so the Gusset is on the bottom. The Gusset looks short at this point, it’s not. You can clip into the seam allowance of the straight (gusset) panel to lengthen the raw edge, if you’d like. I don’t do that.
Remember, this is a three-dimensional thing you are making. So, tuck the Main Panel inside the Gusset, matching the raw edges and pinning as you go.
The raw edge of the Main Panel will ruffle, that’s okay. As long as there are no tucks at the invisible 1/2″ seam line.
When I sew these, I like the panel with curves to be on the bottom. The feed dogs naturally gather fabric just enough to help prevent puckers. But either way is okay.
You can see the panels fit perfectly at the seam line, while the raw edges are clearly a different length.