One of my favorite sites for baby crafts is Make It and Love It. Ashley has some really great ideas and has posted a lot of tutorials. When Oscar started teething a few months ago, I made cloth baby rings using this tutorial. They were a hit (still are).
Now that we are in suburbia and drive a car (bleh), Oscar spends a lot more time in his car seat. The straps leave angry red marks on his neck, so I used Ashley's seat belt cover tutorial to make these:
Ahh, much better. So my point is, check out Make It and Love It. Great stuff.
The measurements in Make It and Love It's tutorial are for a jogging stroller. If you are making yours for car seat straps, consider making them short, or adding button holes like I did.