i have a harmony textbook and right after it introduces you to the diatonic chords in a major key, it gives you a melody and asks you to do 4-part harmony and basically says for every note in the melody think about the chord where that note is the root, and the chord where it is the third, and the chord where it is the fifth, and write the chords down. and then write a bass part using only chords in root position while making sure it works well with the melody. and then fill in the other 2 parts. and watch out for parallel/direct fifths/8ves. dunno, it doesn't sound like a good idea to jump straight into 4-part harmony and be supposed to make the "right" harmonic decisions
you can try what it says, but you must give it the "4 finger test", which is where you listen to it in context and see if it sounds the damn slightest bit good