What sort of sling - a ring sling, a pouch sling or a wrap sling/Hugabub type? I've made em all, plus a few others (MeiTais, Soft Structured Carriers similar to ergo...) If you join up at The Baby Wearer forums they have a DIY section and you will find a wealth of links, patterns, tutes, tips etc.
Personally, I like the ring sling the most out of all those I mentioned as it is adjustable, unlike a pouch. It is simple to adjust and it tends to stay tight and snug, unlike the wrap style sling. A pouch is easy to sew, but you have to be careful to get your measurements right as it is not adjustable. You can buy sling rings online from a number of sellers and if you use cotton drill they are quite cheap to make. I used the Upmama tute.