Hi, Francine. With that much latex paint on the walls--probably really 6 coats plus some primer--you should have no problem. As for the sizing, we don't bother with that either but some people do. In any case, putting on sizing is nowhere near as bad as oil based paint to apply. But do wash your walls before you start with Tri-sodium phosphate (TSP) or a prepared cleaner with TSP in it. Then, rinse well. If you have any spots to fill/spackle, do that next and sand, vacuum and wash the dust up, then spot prime those areas. Then you are ready to paper.
Sounds like you are reading instructions for brand new walls and not walls that have been lived in for a while.