Remember the difference in the two, and just looking at the bottom may help you master this lovely foot. The roll & shell hemmer has a groove that will run from the needle area all the way to the back of the foot. This allows built up threads to flow easily when making a shell hem or rolled hem.
There are many ways to start the hemmer and extra tips for very lightweight fabric. When starting a hem on lightweight fabrics, use a scrap of fabric. Butt it right behind the foot and touching the fabric to be hemmed. Begin sewing and when you come to the fabric to be hemmed continue as usual. Just keeps it from getting bunched up.
To prepare my hem, I finger press the beginning of the hem several inches. Place under the hemmer foot and take a couple of stitches to secure.
Raise the foot and insert the fabric into the hemmer. It will go over the scroll. I raise the foot and hold the back threads with one hand and wiggle the fabric into the scroll with the other. Then pull the fabric backwards until you feel it sort of click into the previous stitches. Now the foot is ready to hem.
While hemming, keep the foot filled with the right amount of fabric. I pull the fabric up and a tad to the left side. This helps keep it filled with the correct amount of fabric.
If the fabric begins to come out of the hemmer, stop. Raise the foot and wiggle back in place, again pulling backwards until you feel it snap into the previous stitches (just like the beginning). Lower the foot and continue hemming. This saves many a tossed garment!
Sheer, lightweight fabric is handled the same, and when going around curves, take it slow.
See that slot in the foot hemmer? Slide lace into it and add the lace while you are hemming. Practice is the best way to master this foot! Once you fill your trash can up several times, then you are on the way!