One thing I've done that's really helpful was to spend an evening with my scrap bin, cutting rectangles that were either 6" x 8" or 5" x 7". I've stacked them in my sewing cabinet, and they've been a great source of pieces for grab-and-sew projects, like this one.
To make your belt, cut pieces of fabric to four times the width of your belt, and whatever length you want. In my case, I have a 1 1/2 inch D-ring, so I cut the fabric 6 " wide (which is what I already had in my sewing cabinet). For the length, I measured where I wanted the belt to sit on my waist, plus 1 " for the D-ring casing, plus about 5-6" extra.
Stitch the pieces together, and trim the seam allowances with pinking shears.
Fold your short edges inside 1/2" and press so that you don't have a raw edge on your belt (obviously I've forgotten to do this in this picture and had to go back and correct myself). Fold the fabric you've stitched in half lengthwise and press. Then fold each edge to the middle and press those in. Finally, fold the entire piece again in half, your belt should be four layers thick now. Edge stitch around the entire piece.
Put both D-rings through one end and fold over. Stitch two rows to make a casing - one at the end of your fabric (as shown) and another close to the D-rings.