If you have any holes, you will want to darn those first. I demonstrate how to darn jeans here. It is basically the exact same technique.
Pick a thread or yarn color that matches and sew in and out of stitches.
For the part of the sweater that would go under the patch, I just mended the hole, quick and simple whip stitch.
I then figured out what size I would want my patch to be on the elbow. I completely free handed this part. I simply cut a piece of paper until I thought that it looked like a good shape/size and used the piece of paper to trace and cut out my leather pieces.
Now you will want to punch holes in your leather pieces.
Walk a stitch spacing tool along the edge of the leather and then punch the holes where you've marked.
Pin your patches in place.
Start the needles side by side, both coming up. Traditionally, you would start one going up and one going down, however because you are sewing leather to knit, you will want the stability of the needles coming up through the leather.
Tuck your thread ends under the patch, they will be covered by the sewing.
Playing follow the leader, each needle will want to pass through each hole.
Here is the inside of my finished patch. I'm going to cover my patch using a really light fusible interfacing. This will keep the rough linen thread off the skin.
Cut your interfacing a little bigger than your stitches and tuck your thread ends in.
Fuse the interfacing to your sweater using a hot iron. Turn right side out and enjoy