To sign letter, hold your non-dominant hand open, with palm facing up, as if you are holding an envelope. Then form your dominant hand into the ASL letter A sign, with its thumb sticking out. Touch your thumb to your lips then press it to your non-dominant hand or 'envelope', as if sticking a stamp onto the back of a letter envelope.
Teach your toddler the letter sign when taking your kid outside to check the mail box. You can also teach it during the holidays, when you show your toddler that you are writing to Santa Claus on their behalf.
Teach your toddler the letter flash card along with the mommy or daddy sign - if your spouse is away on military duty, or with the grandmother and grandfather flash cards, if they still like sending and receiving mail the old way. There is something special about touching and reading a letter as opposed to just sending and receiving email.