" Start with the "root" note of the "I" chord (the note for which the chord is named -- for example, the root of a C chord is the note C) and, on your guitar, count up 5 frets (for example, to the note F)."

Sorry, that's not quite right... but I'm a keyboard player, not a guitarist...

Try "count up 4 notes in the scale" eg C, D, E, F...

You see, there are sharps and flats in the middle there, which will throw out the fret count... at my count, it's nearer 6 frets....