There are many apps and games to help students do this. However, I start my students with extra large flashcards. These cards make it super easy for students to see the lines and spaces. I have found it particularly useful with my students who have learning difficulties - they can point to and count the lines and spaces easily because they are so big. Later we transfer that knowledge onto the smaller staff in their books.
Check out these extra large flashcards in our Etsy Store.