hey...
A guy on Youtube asked this as well and I sent him my own instructions.. weather it helped it or not I don't know, he never replied.. but hopefully it'll help you

here it is:
Okay I can teach you the basics.. it's kinda hard without video or anything.
Basically, you need a cup of water, not full, and a straw.
The first thing to practice is blowing bubbles in the cup while breathing in through your nose. If you've ever spit water out of your mouth, you basically do the same thing.
Fill your mouth with air (and yes, it's good to puff out your cheeks) and close off the airway to your thoat with your tongue. Blow the air out of your mouth using only your cheek muscles, while doing this, your breath in at the same time.
If you can blow bubbles in the cup while breathing in pretty good, try it without the cup and do it again.
Once you get the hang of that, your essentially circular breathing. now to do it on a horn (assuming your learning this for an instrument, if you aren't, just do it in the air)
Breath out, and before you run out of air, fill your mouth up with air and then close off your throat, then do the breathing in and blow out at the same time.
Once you can do this,
CONGRATULATIONS!
you can circular breath.
This is usefull for just about an instrument, except the Tuba, since it requires a WHOLE lot of air.
NOTE: This is just a crash corse, a basic outline of what to do. If I ever get my camera working, I will make a video, but until then, just practice this
If you have any questions, please just ask.
-Smartyy!