Look at the note placement on the musical staff. If it lies on the middle line or above, regardless of which clef you are writing in, the stem will point down. If it is below the middle line, the stem will point up. The only exceptions to this rule are when large pitch jumps occur between eighth, sixteenth, etc. notes, which are linked together, and between chords using a minor second.
Place the stem on the right side of the note if it will point up and on the left side if it will point down. The stem itself should be roughly one octave in length (eight successive spaces and lines on the staff).
Examine any notes that will be linked together. In most cases, the first note will dictate the direction of the stems for the entire passage. However, if a massive jump takes place between linked notes, it may look better to put the stem in the direction of the majority of the notes even if the first note would dictate otherwise. Use your judgment when working with this type of passage.
Place the stem between two notes if you have used any chords with a minor second interval. This method is preferable because writing a minor second directly on top of another note will result in a big, unreadable blob.