Notation is as in the sample above. It's written as 'piano' music: In the treble staff Soprano has note stems pointing up, Alto has them pointing down. In the bass clef tenor has stems up, bass down.

The four voices in four part harmony will generally form a chord at any moment. If looking 'by ear' for a note for your part you can get some ideas by playing a chord (guitar, piano) that is right for the melody (tune) and selecting one of the notes not being used by the tune. It won't necessarily give the best harmony, but it will usually work.

Stu