Singing to your baby is one way in which to provide valuable speech sound practice. One of the first songs you sing to your child should be the ABC Song.
While your child will eventually need to know the letters in alphabetical order, that is not the order in which you will work with the letters when your child is ready to write simple words with blocks or cutout letters.
Capital letters are easier to find available for purchase, but make an effort to acquire or make a set of lowercase (small) letters. Capitals are not the most frequently used forms in printed texts. If you make your cards with a marker, print the letters in the manuscript style used in elementary school.
Some tips for introducing the letters for writing:
Work at first with only a few letters, only one of which is a vowel.
If b is one of the first letters you choose to teach, leave d until much later. Children have difficulty telling them apart.
Teach the child to look at words from left to right by moving your finger under words as you read.