I usually ignore the grooming topics because I just pay for it, which makes no sense as I'm the most frugal person you'd ever meet and I cut my own hair! But after watching a couple groomings, thinking I could learn from them, I learned that I should never try it! I'm not dumb.
But I just called my groomer for you and here's what she does. She breeds and grooms and shows champions.
She uses a 15 blade.
First, while the blade is cool so it doesn't cause razor burn, you do the stomach and butt. You can do these against the grain of the hair.
Second, while it's still cool, you do the top of the head, the cheeks, and the throat. You can also do these against the grain.
Then you do the rest WITH the grain. It seems counter-intuitive to go with the grain as it pushes the hair down. But she is convinced that going against the grain in these areas contributes to the development of skin issues.
In the winter, my dogs get a lamb cut to keep them warmer. The areas listed above under "first" and "second" are cut against the grain with a 15 blade all year long. But for the lamb cut, the other areas which are cut with the grain are cut with a 5F blade.
Hope that helps!