It's easy if the "hair" - probably smaller than that as the viewfinder has a magnifiying glass - is in the outer part - just follow the advice above (take rubber off, unscrew outer glass counterclockwise firmly).
A bit trickier if the dust is inside underneath the lens, in my case it was clinging to the little monitor (by electrostatic) and removing it made the monitor even dustier. It took a while of patient cleaning as tiny pieces remained in the angles of the monitor, in the end I managed to get the rest off with carefully de-fluffed Q-tips. Make sure you are in a clean environment, as dust particles might just plunge inside the piece as you work. After everything looks clean, keep the viewfinder with the closed end upwards (so dust woud have to fly upwards to get in) and dust-blow the piece clean with a dustbrush without touching the monitor with the brush itself.
Following the advice of the thread above you first have to take the handle off, unlatch the tiny fixtures of the electronic cables (don't just pull) and keep track of the tiny screws and remember the way the cable to the viewfinder slips in the tiny slot on the back of the piece.
It took me an hour and a half to get everything out, cleaned and reassembled and cost me a bucket of sweat! But it worked... Good luck!
