I have been wanting one of these forever ....now my little boy it too big! It's ok though, there's plenty of time for more kids ;)
Anyways, I just made this up as I went along so I don't really have a tutorial for it. The one thing I would have changed is the straps that hook to the carseat handle. I made them a bit too long but I might be able to fix it still.
It was pretty easy to make, just kinda time consuming. You could totally do this!!!