7.3Start with only I, IV, and V

In C:

I = C

IV = F

V = G / G7

These three chords alone can harmonize a huge amount of simple tonal music.

So when you first harmonize a melody, do not immediately use all seven diatonic chords.

Try:

C, F, G7

first.

Then add:

Am

and:

Dm

only when useful.