I would recommend reading out aloud and slowly. You'd be amazed how many words we get wrong. Sometimes I would skip words, sometimes I would mispronounce them. But reading slowly helps a lot as it makes me focus on every word and pronouncing it correctly with proper phonation and articulation of the first letter. Another great exercise recommended to me by my speech therapist to focus on the problem areas is that to write down as many words as possible using that particular letter. For me, it's the letter T or D (which require movement of the tongue to fold upwards). Once you have some good amount of words written down, read them one by one everyday. Then you should write a short passage using those words as many times as possible, it doesn't have to make sense, the words have to be used as often as possible so you can learn to say them in a flow.