Once your child has mastered holding a crayon (or other writing tool) and scribbling. You can model writing a vertical line. After modeling a vertical line, make a vertical line with your hand on top of your child's hand. Finally, see if your child can copy your vertical line. The goal is for your child to make an attempt at creating a line that begins near the top of the page and moves toward the bottom (although may not actually reach the bottom).
A fun way to practice copying vertical lines with your child is to give him black paper and a white piece of chalk. The chalk really stands out on the paper and gives him an immediate response to his attempts. (April reminded me on her last post that all children do develop differently. If your child loves the taste of chalk, you might want to try a white crayon or something else.)
Since your child will later learn letter formations by starting at the top of the page, this is a good concept to introduce first.
After your child masters drawing a vertical line. You may want to introduce these concepts as well:
Make your "practicing" fun. Don't attempt to teach your child all of the letter formations. Let them enjoy just making marks on a page and trying out new things.