Hemming a leather coat involves choosing a shorter length for the garment (most coats do not have enough extra leather to add length). Once this is established, measure one half inch longer than the desired new length, fold the leather inwards, and attach it into place using either glue or stitches. Glue is permanent, but it is more convenient to apply than hand stitching would be. You can also hem leather using a sewing machine.
Try the coat on and mark with chalk where you would like the new length to be. After removing the coat, lay it flat. Add a half-inch to the mark, then fold the leather inwards with a ruler to ensure an even length, lining with chalk or a marker as you go. If the coat will be more than a half-inch shorter, you will need to cut the leather with scissors all around (leave a half-inch for the hem).
Attach paperclips, tape, or weights to the inside of the coat to hold the proposed seam in place, using the chalk or marker lines as a guide.
If you are going to use glue, open the container and apply glue to the brush. Brush it onto the half-inch section of leather below the marked line on the inside of the coat and press it down as you go. Place some books on top of the seam while it dries.
Use a leather needle and rayon thread matching the color of your coat if you are going to hand sew or use a sewing machine to apply the seam. For either technique, you can use a straight stitch of up to 3.5 inches in length about 1/8 inch from the top of the folded leather on the wrong side. It may help to use baby powder at the top of your sewing machine work surface to help the leather slide along.