I know this is an old thread, but I can help. I just finished my "ktan" and it works with my 1 year old nephew.
Here's what I did:
1. Using a soft measuring tape, measure from the tip of your shoulder to your side just under your ribs. Double this measurement and add 2" for a seam and 2" for shrinkage. This is the length of fabric you need to buy.
2. Go to Joanns/Hancocks and buy a stretchy fabric of your choice. Don't go for 100% polyester unless you want to be really hot really fast. Also, don't buy something that's super stretchy. You want almost no stretch along the length and a moderate amount of stretch across the fabric. I love their 60% cotton/40% polyester interlock. Go for something that's 60" across too. This will give you enough for the two rings and the support sash.
3. Take fabric home, prewash and then cut into 3-20" pieces along the length. You should end up with three pieces of fabric 20" wide (stretchy) and your measurement from #1 long (non-stretchy).
4. Take one piece and sew it together along the short ends with a French seam (gonna have to look that one up, don't have the room to explain it). Repeat with a second piece. The loops are done (unless you want to hem the sides- I didn't). I didn't make the smaller connecting loop because the ktan is actually easier to use without it.
5. Now the support sash. This uses the third piece. It isn't going to be long enough to tie in the back, so you need to either sew on some fabric to the ends to extend it enough to tie or do what I did: Buy two rings like you would use for a ring sling and sew them onto one end (just like for a ring sling). You use it just like normal except that instead of tying in the back, you thread the free end into the rings like a ring sling.
6. Download the instructions and start wrapping.
I love, love, love this sling. I started with a pouch (hell on the shoulders), a ring sling (ditto on the shoulders) and then wraps (loooooved, but too much time). This sling works just like a wrap, but without the tying.
Note: I don't know how long you can use this wrap. I carried my 1 year old nephew in the front carry position and it worked really well, but I don't think the cradle or kangaroo would work at all with the stretch of the fabric. I think a lot depends on whether your fabric stretches along the length. If it does, then the wrap will really sag with a heavy baby.
I know it's late, but better late than never!