It is that situation where the raw hem edge is wider than its final location. This happens often with an A-line skirt or dress. The upper edge of the hem wants to pleat or tuck if pressed into position.
One approach I see in pattern instructions is to run a gathering line along the raw edge and ease it into position. I think that will work and might even end up looking pretty good. But my preference is to hem it with a bias strip.
This approach is similar to creating a facing for the hem. With a true facing you would need to create the same curve in the facing as in the hem edge. No need for this with bias. Bias will re-shape itself as needed with just a little steam.
Cut a bias strip of fashion fabric at least as long as the hem edge. I like to cut it 1.25 inches wide. Much easier than drafting a facing, as you only need to know how long to cut the bias strip. Cut it the length or circumference of the hem plus an inch or two. It's usually easy to piece the bias pieces if your remnants of fashion fabric are too narrow. Or you can choose a contrasting fabric.
|My fashion fabric is solid black on the right side, striped on the wrong side, hence the obvious bias stripe on the wrong side of the bias strip.|
|*In this instance, I placed the wrong side of the bias strip next to the right side of the garment hem edge so that the bias stripes would be visible on the inside of my garment. In most cases though, you would place right sides together.|
|Note that the bias strip is next to the feed dogs, garment is on top.|
Once these are sewn together, press the seam allowance toward the bias strip. Then fold the bias strip to the wrong side of the garment, and press again. The fold should favor the garment side in order to keep the bias strip from showing on the outside.
|Wrong side of garment just after sewing bias strip to hem|
|1/4 inch hem pressed toward bias strip|
|right side of garment, pressed to favor the solid black side|
|wrong side of garment, pressed to favor the solid black side. looks like a tiny piping here|
|Finished hem in sample (wrong side of garment)|
|Finished hem in coat (wrong side). (I love the Linda Lee approach to mitering.)|
HTH! Happy sewing.