Summarize:

You can trill your tongue by holding your mouth open and placing your tongue either on the roof of your mouth or allowing it to rest with the tip touching the back of your bottom teeth. Take a deep breath and vibrate your tongue. It should sound similar to a cat purring.  When the tongue is full of tension and in the wrong position, it can block the tone and cause the sound to be swallowed—making you sound throaty. A relaxed tongue will allow the tone to flow freely and sound more clear.  In general, it is helpful to release the tension in your entire body before singing. After doing lip trills, sing a note softly, then slowly increase the volume. Practice getting softer, louder, then softer again. This exercise can help you stay on pitch while also strengthening your breath control. Point your chin about 1 inch (2.5 cm) in toward your chest and down toward the ground. Keep your chin firmly in place as you sing the notes of an Ah scale.  It may feel more natural to lift your chin upwards when you are projecting your voice. Try to avoid this—it may work temporarily, but can cause vocal problems in the long run. Keep it in place to gain more power and control and eliminate strain on your vocal cords. Practice in front of the mirror to make sure you have the correct position. A lowered soft palate creates a nasally sound when speaking or singing, which affects the clarity of your words. You need to create space at the back of your mouth by raising your soft palate in order to produce a clear, rich tone.  To find your soft palate, run your tongue across the roof of your mouth. The hard part is called your hard palate and the fleshy bit towards the back of your mouth is called your soft palate. To raise your soft palate, yawn or laugh. That feeling of space in the back of your mouth is your soft palate being raised. You should have this feeling every time you sing. Take a look at this YouTube video of someone singing inside an MRI machine to see what happens inside the body when you're singing: https://www.youtube.com/watch?v=J3TwTb-T044&t=157s. You should feel the vibration in your head, face, and lips. If you feel this vibration while you are singing, it means that you are resonating effectively. Proper resonance and vibration allows you to produce a clear tone with a strong projection without using excessive force.  If you don’t feel the vibration in your face while you are singing, keep practicing your tongue trills and make sure your chin is pointed in and down while you are singing. For additional help, try slowly shaking your head from side to side while humming.
Warm up with tongue trills to release tension and improve your tone. Control your breath flow by singing long tones. Keep your chin pointed downward while singing. Keep your soft palate high and your tongue forward to get the right tone. Check for a vibration in your face to confirm you are singing correctly.