Research diction tongue twister exercises associated with various letters. For example, Write-Out-Loud.com suggests repeating "six, thick thistle sticks," to help learn the proper diction for enunciating the letter "S."
Write down multiple tongue twisters for each letter you want to practice.
Repeat the phrases slowly and deliberately several times a day.
Increase the speed in which you repeat them as you are successful at correctly pronouncing each word in the phrase; however, maintain a pause between each word so they do not become jumbled together.
Practice proper breathing to help the smoothness and fluidity of your speech, which can also enhance your diction. Start by breathing in and then out to counts of four. Gradually increase the counts as you practice until you are breathing in and out to counts of 10.