A fun way to practice writing letters is to make some pudding. Put the cooled pudding in a ziplock bag and then put it in a second ziplock bag to prevent leakage. Show your children how they can use the bag to practice writing letters.
This is great for kids who have tough control with a pencil. They use their finger to write the letters. It's helpful to show the child how to write the letter first, then let the child try.
Stay with your child so you can keep modeling to encourage correct letter formation. Remember it is best to start from the top of the letter to the bottom of the letter.
Children often want to start at the bottom to write the letter, but this actually takes more energy than writing from the top to bottom on the paper.
Also encourage writing from left to write when the child makes letters that go across the paper like the letter "E".
This activity can also be done with children's paint in a ziplock bag or you can use shaving cream on a cookie sheet. These are all tactile ways to practice making letters.
Don't worry about writing correctly on lines until until Kindergarten/First Grade . It takes a lot of fine motor control to be able to write on lines with control and correct letter formation.