There are many varieties of feller/folders. Some turn the fabric edge down one time, some turn down a double fold, others turn the fabric edge up, and still others will create felled seams between two pieces of fabric as you'd see on jeans, menswear shirts, etc. The photo above shows many sizes of clean finish, or double fold, feller attachments. In general, for home coverstitch machines, you'll use the single downturn feller to create topstitched hems with a covered raw underside edge, as shown below. Fellers attach to the bed of the machine with thumbscrews. Some of the larger fellers (like the 1" feller below) can only be attached with one thumbscrew. That's OK -- just tighten the screw enough so that there's no play or wiggle in the feller. It will take a little bit of experimentation with scraps to learn where to position your feller so that the underside of the hem is coverstitched without leaving excess to trim away. Once you have this position, keep the feller attached and remove the fabric. Use a Sharpie marker to mark the needle positions directly on the feller as shown below. From that point onward, you'll now be able to quickly line up your mark with the needles or needle markings on the foot and no more experimenting will be necessary. You may also want to mark the thumbscrew location onto the attachment. If the screw holes in the bed of the machine do not alllow any further side-to-side adjustment, you can reposition the feller on the attachment bracket after loosening the metal screws just to the left of the white thumbscrew in the photo below. Once your feller is positioned as desired, don't forget to tighten those screws again. To begin, finger or iron press a "starter" fold about 2" long and the width of the feller spec. My examples show a 1" feller, which means the finished width of the hem is 1". So, I would finger (or iron) press under a 1" width along the first 2" of the fabric. Lift the presser foot and slide your raw fabric edge into the attached feller and allow the feller to turn the edge of the fabric the amount you marked. This sounds more complicated than it is. The feller will do most of the work with only a little guidance from you. You may need to slide the fabric back and forth under the presser foot a couple of times to seat the fabric into the folding curls of the feller. Once the edge is seated, pull the fabric under the foot and put the foot down. Start stitching. The foot and feller will hold the fabric in place with only minimal guidance from you to keep the fabric from "dragging" in your lap, which could distort the felling action. With a properly positioned downturn feller, this is how the fabric will come off the machine. The hem is topstitched on the right side and completely covered by looper stitches on the wrong side, with no excess to trim away. You can see a little bit of distortion as my fabric is leaving the feller. This is because I'm trying to stitch and take photos at the same time and the fabric is dragging below the machine bed. I needed my third hand!