Photoshop :: Cropping To A Specific Size

I hope I am able to adequately describe what I want to do!

Back when dinosaurs roamed the earth and people used film in their cameras, here is how I used to mark my clients' photos for cropping:

1. A photo proof would be on the table.
2. I would put a cardboard device over the photo.
3. With the device I could make a rectangular window of various standard frame sizes, like 8x10 or 5x7.
4. I would put the "window" over the photo wherever it was most pleasing, and then, with a pen, mark the outline of the area to be kept.
5. The photolab would then crop the photos accordingly.
6. The photos could then be printed to fit perfectly into frames.

Whew! Does that make sense?

Now: How do I do the equivalent in Photoshop?

I do NOT want to simply "resize" the photo. I want to CROP WITH THE FINAL SIZE IN MIND.

It seems like you could do this with the crop tool, but I can't figure out how.