Here's what you need:
27 inches of 7/8'' grosgrain ribbon
7/8" elastic to fit around a cup plus about an inch to sew to the ribbon (I used 9 inches total)
Fold under one end of the ribbon about 1/4 inch (just to seal the edge), sew it with a tight zig-zag (remember to back stitch!). Then fold THAT part over itself again about 1- 1/2 to 2 inches (to create a loop). I sewed on the same line of stitches that I had just sewed. You can see the loop in the picture below.
Then fold the elastic in half and sew about 1 inch up from the end (right where you want it to pinch together around the cup). Then sew the extra inch or so of elastic to the ribbon in a few spots, I did 3 spots on the green one, and 4 on the pink one (I always go back and forth over the stitching just to make it's really secure since this will get all the pressure). And you're done!
Loop the leash around the handle of a stroller or the base clip of the car seat (I put mine on the bottom clip of the 5 point harness so it's between their legs) and put the elastic end through the looped end and tighten to secure. Then put the elastic around the cup. I've found this size fits most cups pretty universally, even some that don't have a smaller section in the middle.
Now when your kid chucks their cup you can always get it (and eventually THEY can get it themselves). It has saved me HOURS of crying in the car (from the girls, not from me. Hehe!) Sometimes I even get smart and string their snack cup holder onto the leash before I put it on their cup as an all in one snack leash.
Hope this helps a momma to regain some sanity. :)
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