I personally would purchase the pixie tutu instructions from hadamos here: http://hipgirlclips.com/forums/how-m...tructions.html
These are by far the best tutu instructions I have seen free or paid. It is for a 3 layer tutu, but you get the basics for all tutus here (measurements, lengths, etc).
I use a square knot as opposed to a slip knot (personal preference). I'm sure if you search "reversible" tutu you will get a better explanation of the up/down method. I learned this from a different tut that I purchased, but that was the only thing I got out of it and it honestly wasn't worth the $$ I paid for it. For that reason I won't post a link to those instructions.
Good luck! Once you get the hang of it you think, "Why did I think this was so hard?"