I've used this pattern to make my own slings -
http://www.ida.net/users/stace/images/sewsling.jpg

I just use about 1.5m of cotton fabric (2 pieces, one is the 'outside' layer, the other is the lining)

I've added a pocket on the side so I can throw in my keys, licence card and phone - so it's super easy to take Jasper out to the shops really quick if needed. The pocket is about 12cm x 5cm with some velcro to close/open it