The first letters I introduced to my daughter were the letters of her name (2 at a time). We use sandpaper letters (lowercase) to learn/say the sound of the letter and trace the letter with the index and middle finger. After practice with this, I introduced letter writing in sand (or cornmeal) in a pie plate, first tracing and saying the sound and then writing it.
Blank Slate Chalkboard and Wooden Display Stand (Large) from Montessori Services Alphabet cards from The Wooden Wagon.
She is now practicing name writing on the chalkboard and using some new alphabet letter cards.
She also likes me to join her with this. We use the letters she has been introduced to, taking turns choosing one, tracing it, saying it's sound and drawing it on the chalkboard.