Ken,

I believe people cut a hole in the plastic bag for the lens.  You can also
use a cheap shower cap to protect the top of the camera from rain.

Don

Cut out a hole for the front of the lens! Then loosely fasten the bag to the
lens barrel with a rubber band.

---Bob Gross---

   Those I have seen, and one time I used one in a rain forest, use a heavy
duty large bag and make a hole in it which is carefully taped to a UV
filter.  The filter is attached to the lens and the camera sealed in the
bag.

Shooting through the plastic would be bad in my estimation. However, if you cut
a hole in the bag that the end of your lens can fit through it will work much
better.

Dave

Try a plastic freezer bag by cutting a hole on the bottom smaller than the
lens hood, put the camera into the bag and then pull the bag over the lens
hood.  The plastic will stretch and make a tight fit around the hood.

Stay away from the heavy-duty slider zip-loc type bags - the zipper plastic
is very stiff and makes using the camera awkward.  The larger the bag the
easier it is to use.  Take several spares along - they are cheap insurance.

This method worked well for me in Alaska last spring - there was frequent
rain, but I managed to shoot successfully anyway.

Bob in Las Vegas