Perhaps a day late?
Anyway, in case you look at this again, I had a kari-me when ds was teeny, and managed to bf in it just fine (first time i did it i was doing a great long unplanned walk passing lots of building sites with builders having their lunches on walls and watching and it was just fine. he was about 3 weeks old then)
I normally carried him in the 'cuddly' hold, I think it was called, so where he sort of lay down in front of me in a curled up position, looking upwards, and so if he needed to feed I just wriggled my clothes around, turned him inwards, and did the sling up a bit tighter. I found it hard to bf when he was sitting up (never tried when he was teensy), which is partly because I have the smallest boobs in the whole world, even when newly breastfeeding. But then I found the trick was to get his bottom a bit lower than seemed comfortable (slacken the sling off a little) and again just wriggle about until nipple gets into mouth.
If you want to get him from a sitting to a cuddly hold, then at least for the first couple of times you probably want to take him out of the sling altogether and put him back in, just to make sure he's in securely. Once you've worked out the angles of it all, you'll just be able to slither it all about. But basically in the sitting hold, they sit across the crossed fabric, and in the cuddly hold, their head is supported by one cross, their feet by the other, so you need to get them into one 'arm' of the sling. I can't explain well, but I could look for a better explanation if you want!