I began with the same size soft piece, 30" x 36". The exterior piece should also be that size.
I measured the distance between the straps on the carrier and the sides of the stroller. The carrier straps were 12" apart and the stroller sides were 14" apart. I added 2" for seam allowances and a little bit of room. 3" - 4" would have worked also, but I didn't want it to be too loose on her carrier.
With the 30" x 36" soft piece, fold it in half and mark your distance from side to side. I made mine 16", so I marked 8" (folded). I cut a straight line 8" from the center to 10" down (marked on fabric) and then made a diagonal/curved line to the edge of the fabric. The picture explains it better.
Unfold your fabric and lay it on your exterior fabric, then pin right sides together. I left the exterior fabric larger an cut the excess after sewing.
Mark a 6.5" x 6.5" square in the bottom corners and cut out.
Separate the bottom edges of your pieces and pin them with "right sides" together, so the seams will be hidden after it is all together.
Sew the corners together where pinned and then tuck one corner into the other. Pin the two fabrics back together.
Mark 10" down from the top and leave about 1/2" of space for the elastic to later be sewn in. You could go ahead and sew in your ribbons at this point (see other tutorial for how to place them). Leave a space at the bottom of the blanket for flipping the fabric, then sew around the two layers. Be sure to leave a 4" space on the bottom edge for flipping and a 1/2" space 10" down on each side for the elastic.
After you sew the layers together, cut the excess and flip your blanket right side out. Then sew your interior stitch about 1/2" in from the edge, all the way around the blanket. Make sure you close the hole you used for flipping the blanket.
After the interior stitch is done, thread elastic through your channel (bottom edge of blanket) and pull until it is the tightness you're looking for.
This is what my blanket looked like after pulling the elastic tight. Pin the elastic in place, and stick your ribbon ties in the gaps before sewing. My ribbons were each 24" long.
Stitch over the elastic edges and the ribbon to make sure they're secure. I used zig zag and straight stitches to make sure they were secure. Your blanket is now complete!
Here it is on my carrier and the stroller. The elastic edge will keep babies legs nice and toasty.