There are a couple of common problems.  Most patients expose too much
floss--over maybe 3/4" you lose control, which leads to snapping the
floss into the gum and a bloody mess.
    Also, patients often use several fingers of each hand to manipulate the
floss, and this means often there isn't room enough to reach the back
teeth, esp. the upper molars.
    What I recommend is that patients wrap the floss around their middle
fingers.  On the upper back teeth, I direct the floss directly over the
pad of the thumb on the same side.  Do not open your mouth all the way
up--this actually decreases the space between the upper molars and the
cheek (if you don't believe this, without floss direct your index finger
back along the gum toward the back teeth and slowly open your mouth).
So with your mouth halfway opened and the floss balanced between the pad
of the thumb on the same side and the pad of the index finger of the
other hand, with only about 3/4" of floss exposed between, GENTLY saw
the floss between the teeth until it passes through the contact.  Now,
stop "sawing", and pull the floss down and against one of the facing
surfaces of the teeth to the contact and back to the gum.  Do the same
for the other tooth surface facing the space.  Remove the floss, expose
a clean section, and repeat.
    When you get to the midline, reverse your hands (ie: your other thumb
and forefinger will be used).
    On the lower teeth don't use your thumbs.  Usually I will use both
index fingers.  Otherwise the technique is the same.
    It is always better to see this done.  Ask your dentist or hygienist to
demonstrate.

Steve