Capture the spider carrying the egg sac in a clear jar or tub, like those you buy cold things in at the grocery store or a medium-sized Chinese restaurant soup takeout container (after cleaning it out).
I recommend plastic instead of glass so that it won't break if you make any sudden movements etc.
Slide a paper under the container as it's against the wall/ceiling/floor, so the big spider climbs on to it with the egg sac.
Then put something stiffer (like a thin hardcover book or clipboard) under the paper to help keep it flat against the bottom of the jar, and carry the mother and her sac all together outside. Set it all down, tip over the jar and if necessary gently tap to encourage the mother to climb out.