Saying the first part of a word is most difficult. So by practicing basic words. By saying the first letter naturally. Most people will begin to speak more clearly.
It has to deal with doubts, fears, and hesitation. When an individual learns to say the letter correctly. They naturally follow through with the full word. With a natural reaction. These techniques are used in speech classes. To help people with speech problems. Naturally as adults we let our nerves, emotions or patience interfere with our ability to speak clearly. That's why even native speakers calling an emergency number. Are asked to speak calmly and slowly. So emergency works can precisely understand the phonetics.
Actors, news reporters, and many more are required by their careers. To be able to use basic phonetic in delivering their content to the audience clearly. After phonetics are used. Then accents can be altered or modified more quickly. To suggest tone and pitch. To sound like a completely different person.
A: how to say words beginning with the letter A. Keep your tongue relaxed at the bottom of your mouth and move it accordingly to the second letter in the word.
Adam,Apple,Ant
B: How to say words with the letter B.
Roll your lips together like your putting on lipstick and push the word forward.
Bat, Boy, Ball
C: how to say words beginning with the letter C. Slightly open your mouth. Pull back with your facial muscles similar to a small smile. Then follow the second letter in the word for pronouncing it correctly.
Cat, Cold, Can.
D: how to say the words starting with the letter D. Place your tongue just above your teeth pull your tongue downward while saying the word.
Dog, Den, Dig
E: how to say words beginning with the letter E. open your mouth slightly. Pull your tongue back while saying the words.
eel, eat, egg
F: how to say words beginning with the letter F. Roll your lower lip under your top teeth and roll out the word with your lower lip.
Fun, Fit,Fan
G: how to say words beginning with the letter G. Mouth open slightly. Roll both lips under slightly. And push out the word.
Gel, Gap, Gun

This a small example how to speak clearly. Using small words to things your use to in your every day life. Whether it's person's name , activity, or subject. Will greatly increase your fluency in speaking. Small words , small 3 to 4 word sentences of everyday life builds confidence. From there you can greatly increase your vocabulary and then use larger sentences. With more comprehension of what your trying to deliver in your speaking.
As infants we learn to communicate through speech. From there simple sentences, then we increase our vocabulary, then we work on grammar.
Think like a child, speak like a child, grow like a child naturally. As adults we try and force ourselves to fast. Then become frustrated easily. That's where we want to give up.
Humor, music, and everyday things are the best tools to use while learning any language. If you have to use the bathroom that urgency forces you to speak clearly. In every language we learn to speak by our needs first. They are the single most critical event we face daily.
From hunger to thirst and more. We encourage children to do the same.
From basic communication we're able to adapt faster than by any other means out there. A great example of this is your placed in a foreign country. In which you do not speak their language. Whatever your basic core needs are at that moment you will quickly adapt to learn out of survival. It's your primal need to communicate. I hope this helps you.
There are hundreds to thousands of ways to effectively communicate. The trick is finding the way that best suits you. Builds your confidence.