wraps, ring slings and pouches are the easiest for DIY'ers to make. Mai tais are also do-able, but not as easy, and there are so many variables that effect the way a mai tai fits and works that it is hard to get it just right the first time.
some wrap fabrics require nothing more than cutting it to the right size, others need the edges hemmed. Ring slings need rings (get them from slingrings.com) , and you need to decide on a shoulder style (many people like jan's instructions here: http://www.sleepingbaby.net/jan/Baby/sling.html ) There are different ways of attaching the rings to the sling, and they all feel a little different.
Pouches are pretty easy to make, but you need to know what size you need. A proper fit is essential when wearing a pouch sling. Jan has instructions for pouches:http://www.sleepingbaby.net/jan/Baby/tubesling.html
this is one of the first pouch making pages I remember:http://www.ida.net/users/stace/sling.html
register at the babywearer, there is a lot of DIY info and a DIY forum, all about making baby carriers:http://www.thebabywearer.com/lists/Sewing.htm
it's important to use strong fabric (no light quilting type cottons or calicos) and good thread. If you use stretch fabrics, pay attention to which way the stretch goes when cutting the carrier--the direction with the most stretch should always be perpindicular to the force of gravity.