I would like to know, suppose a clef with key signature (e.g., G is always sharp), independent of octave, appears at the beginning of the staff.
Then every G that appears must be read sharp.
Then what do I write, to have the reader read a normal G, or a G flat?
Place a natural or flat sign (an "accidental") immediately to the left of the note to be changed.
The below image shows examples of key signatures with G-sharp where the notes themselves are changed to G-natural or G-flat. Once changed, the G on that line or space will remain natural or flat until the end of the measure. After the end of the measure, the key signature takes over. So if another G-natural or G-flat is desired, then another natural or flat sign will need to be placed.
Accidentals affect only the specific line or space on which they're placed. This is also shown in the image below.
When there's a sharp in the key signature, say, on the G in the treble clef, it means every G in that piece will be played as a G♯.
To cancel that, there needs to be a natural or flat sign just before the note affected. That will then last until - the end of the bar it's in, or - it's changed by another accidental to say what it'll be next. But regardless, in the very next bar, it will need to be played as G♯ again - unless affected by yet another natural or flat sign.
Those accidentals will only affect notes in the particular octave tey're written in - any other G notes in other octaves will still be G♯.
