Harmony notes are always controlled by the chord symbol, not by the name of the melody note.

Let us now have a look at a C melody note but this time with the F Major chord.????

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The first thing to decide is what chord notes could be added to this melody note to obtain correct harmony.? This is controlled by the chord symbol.? As it is F, you must only use notes from the F chord and no other.?

As you realize, you might sometimes find a C melody note with the C chord, and in that case you would choose harmony notes from that chord but in this case, whilst the melody note is C, the chord symbol is F, so harmony notes must come from the F chord, (F, A, C).?

You could choose the A for close harmony or the F for spaced harmony or both.

Remember to always add harmony notes below the melody note never above it.

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Adding G Chord notes to a D harmony.?

Your guide notes, should you wish to use them, are G, B, D.?Play the melody note D and chose the other two from the G chord, B for close harmony and G for spaced harmony.