Always use a straight edge for stems and beams. Beams should be thicker than stems.
Stems should extend an octave above or below their notehead. For the middle line and higher, stems go down; below the middle line, stems go up (for ledger lines and beamed notes, these rules can be relaxed so the beaming looks good).
Accidentals extend above or below their notehead by a perfect fourth.
Plan how many measures you'll have on each staff system, so you don't run out of space.