It takes a lot of hand eye coordination, practice and muscle strength before kids can learn to write. You can start teaching them these skills by having them trace their finger along a path, trying to keep their finger between the lines.
After they've mastered this, have them trace the path with a pen or pencil. Then have them trace a path that makes a letter.
Kids like new things so instead of tracing on paper, maybe try having them connect dots to a letter you write on a white board.
There are many different things kids can do to practice their letter writing. Have them trace their letters in a pie tin filled with a little colored sand. Make letters out of playdough. Make pudding and have the child write letters with their fingers in that on a cookie sheet. Trace letters with string. Print and laminate large letters and have the child trace them with a toy car.