To introduce letters for phonics practice, I use flashcards for the single letter. I show the flashcard of the letter A for example and on the back it has 2 things that start with that letter (with the words written out by the object.) You could just draw the letter and two objects on the board and do it that way.
First, show the letter to the students. We'll point out that the letter is called "A". Then, show them the objects that start with the letter "a", "apple" and "alligator". Underline or write the word on the board with the initial "a" in a different color. We want to draw attention to the "a" at the start.
Read the words repeating the "a" sound several times and then the full word:
"a" "a" "a" "apple"
While you are reading point to the "a" and ask the students what sound "a" makes when we read it?
Repeat the steps above and quiz the students for the answers.
As a quick activity, I like to have an alphabet hunt. I hide foam letters around the room and students have to find the "a"s. Once all of the letters are found, students who found them bring them to the front. I ask them
"What are 2 things that start with "a"?
"How do you read "a"?
Then I give them a sticker for each "a" they found if they can answer the questions. If they can't answer, they have to ask other students and come back with the answers.
That's it. Quick and simple single letter introduction. Then next lesson I do a new letter.
You can follow up with worksheets on that letter, which I do as homework.