Try to avoid parallel fifths and octaves and try to keep at least one voice in each progression moving in the opposite direction than the others or stationary. Also avoid using S chord after D chord.
As you are writing always be conscious about the notes of the Tonic (F, A, and C), the Subdominant (Bb, D, and F), and the Dominant (C, E, and G) chord in F major.
This means that F in the melody could mean S or T chords, G - D chord, A - T chord, Bb - S chord, C - T or D chords, D - S chord, and E - D chord.
Here is the PDF file for printing. After you have completed the exercise, play it on the keyboard to see if you like your harmonization. Post your time to comments.