A first step to helping your child write letters is to build on your child's interests. Choose one of these words as your first target. It is best to choose a words that begins with a consonant. A second consideration is your child's motor development. Frustration will build in your child if you focus on a inappropriate skill.
Most preschoolers find straight line letters easier to form.
A third consideration is simplicity. It will be easier for your child to succeed if you provide him with a plausible task such as writing a t' rather than a more complex g'. Once you have chosen your target word and letter, look around your house for some examples of this word. Perhaps your target letter is t' and your target word is tooth'. Find or tell your child's favorite tooth' story or have your child tell it to you.