The biggest thing to remember is to keep your back
straight. Keep your back erect (if you're in a chair it's
best not to rest your back on the back of the chair), and keep an
upright posture. This keeps you alert, and allows you to
concentrate more easily on your breath.
Your eyes can be either closed or open. Again,
the goal of this whole "meditation" thing is to work out your
attention muscle. If you find you can concentrate better on your
breath with your eyes closed, as many people do, then it's probably
best to keep them closed. If you're tired and find yourself dozing
off when you close your eyes, try opening them slightly and
focusing your gaze softly on a space on the floor in front of you.
For me, this becomes distracting, so I keep them closed and only
open them if I'm tired.
Don't worry about your hands. Some people like
to form circles with their thumb and another finger, but that
doesn't really matter, in my opinion. I usually just rest my hands,
palm down, on my legs, wherever they feel the most
comfortable.
Cross your legs however you want. I usually
cross my legs in front of me, and I think that works fine for most
people. If you want to fold your feet like a pretzel you can, but
if you're use meditation to strengthen your attention muscle, it
may be easiest to keep to a simple, cross-legged pose.
Look slightly downward, even if your eyes are
closed. This opens up your chest. Again, though, find a place
that's comfortable - one that keeps you upright and opens up your
chest at the same time.
The biggest point I can make about how to sit is to
find a pose and posture that's both comfortable and keeps you
upright. The guidelines above work best for me and most of
the people I know, but they may not work for you. The most
comfortable meditation pose will give you so little alertness that
it will put you to sleep, and the least comfortable pose will keep
you alert, but at the expense of your comfort. The best advice I
can give is to try to find a place in between that works the best
for you.