To find the length of your belt it's best to measure it around where it would sit on your jeans. Since most jeans sit lower on the waist these days, your belt is going to be bigger than your natural waist measurement. Just wrap the ribbon around and add about 7 - 8" to fold back and go through the d-rings.
I used 7/8" pink solid grosgrain ribbon with 5/8" wide heart printed grosgrain ribbon. I centered the printed ribbon over the solid ribbon and edgestitched both ends down. Giving the ribbon a good press with steam took out all the puckers (stitching ribbon makes it really puckered, ironing it always saves the day). The d-rings should be about 1/4" wider than the ribbon (I used 1 1/4" wide). Thread the ribbon through both d-rings and stitch down. Be sure to double bendback the ribbon so no raw edges are showing. Grosgrain ribbon frays like crazy. Double bendback the other end of the ribbon and stitch down.
You can get as creative (or uncreative) with this as you want. You can use a few layers of different sizes or just one ribbon... one thing I have learned is that it's always best to stitch the ribbon down. The heat adhesive tape or glue just doesn't look as nice and doesn't hold up as well.