Watch the How To Use These Exercises online guitar lesson by Karan Andrea from Guitar Player Wanted: Vocals A Plus
As you work with your vocal exercises, think about the muscles involved. Pay attention to where the sound is (or should be) resonating. As you do this, you will learn to apply the knowledge you have of your instrument's anatomy to these exercises. Then you will be able to apply these techniques to the songs you have to sing. As you build good habits and create muscle memory, these actions begin to happen naturally. Then you will feel more confident, and you will sound terrific.
Practice until you are keeping your throat relaxed, and you are keeping your larynx in a natural position without thinking about it. Use the zipper image to help train your vocal cords to work smoothly through your break, until you are generating a nice 'mix' of chest and head through that area of your range. Learn to coordinate dropping your jaw and raising your soft palate, so that this just happens naturally as you sing higher notes. Take a breath between each repetition of each exercise, and make sure your breathing and phrasing work together when you are singing. Practice keeping your ribcage and back out even on your exhale, while pulling your abdomen up. Practice quick 'refills.'
Some of these things you can do anytime - raising your soft palate, or keeping your back and ribcage open - those you can do at work, while driving, walking, anytime. By just doing these exercises, and by really understanding your instrument, you will become a better, and more confident singer than you were yesterday.
You can do these exercises every day if you want. I would suggest you do them frequently at first - at least every other day - to create that muscle memory you are after. After a while, and you will know when, you can do them less frequently. But please - warm up before every rehearsal and every gig. You can use the a cappella exercises by themselves as a pre-rehearsal or pre-gig warm-up. I can't stress this enough. Always warm up before you sing for any length of time. You probably warm up your hands before you play your guitar, particularly if you are a serious player. Athletes warm up before they practice or compete. These are muscles, don't forget. Warming up isn't about 'listen to me, ain't I great' - it's about getting your muscles ready to perform.