The last seam is the most fun but you can do it.

For the handles you will need two pieces of batting 1-1/4 inches wide and 40 inches long. You also need four pieces of fabric 1-3/4" wide and width of fabric. Yes, that a bit extra. I use 2 from the outside and 2 from the lining fabric. Place the fabrics face to face and lay the batting on that. Pin it together then sew your quarter inch seam along the batting on three sides, leaving one end open. As you sew, stretch the batting into the path of the needle and stitch is down. This will make it easier for the next step which is to turn the straps right side out.

Decide whether you want the straps on the points or in the valleys. Pin the straps to the bag and decide where you want them and how long you want them. I like mine in the valleys and I like to be able to pick up the bag and sling it over my shoulder without it catching on anything.