How Should I Practice While Studying Voice?
The "WHAT" you practice is often less important than "HOW" you practice for voice lessons.
Balancing the contradictory demands of relaxing and activating muscles (at the same time!) requires honing a super keen sense of bodily awareness.

So, HOW, should one practice?
By C O N S T A N T L Y doing these three things:
Listening to your sound (regularly recording yourself and listening to yourself is a great idea and SO HELPFUL; it’s not narcissistic, it’s a learning experience).
Monitoring your body for any signals of pain or tension.
Monitoring for any feedback signals your brain might give you [Like, “error messages” sent by your brain - if you were a robot your brain might say “Error! The output of this singing technique is NOT matching the intended result. Should I troubleshoot?” Tell your brain “Yes! Run the troubleshoot program!”] Focus in on the trickier melodic lines, or the tricky warm up exercises. Do them again and again, and do something different each time, experiment! Reflect and consider what you are doing differently each time, search for the “cause and effect”, and change your habits.
Always, always, always, be thinking about WHAT YOU ARE FEELING:
"What is my body telling me while singing?"
"What am I emotionally feeling right now?"
Let's talk about physical bodily awareness first:
When singing be on the constant lookout for ANY sensation; put on your sensation radar.
Possible physical sensations to look for:
The several layers, inner and outer, of muscles all working together (from the lower and upper abdominal area, the inner pelvic and hip area, the entire ribcage); the feeling of what it takes to maintain expansion and finding the "gas pedal" that "adds gasoline to the engine" of your voice box.
Look for any tension in the shoulders and arms, or any tightness, shaking, or nervousness that is restricting your breathing
Think about the angle that your jaw is opening; exactly how wide is your mouth open?
Check how much you are smiling and the exact shape of how you’re smiling
Does the shape or position of your tongue, cheeks, lips, and soft palate remind you of anything else you do in life? You'll need a way to memorize certain mouth shapes. Certain vowels and consonants in singing actually mimic actions in life other than singing - you can replicate them accurately by imagining those more familiar actions! For example, eating an apple is an everyday action (if you want to keep the doctor away, right?). To sing open vowels (like "ah" as in father, or "ih" as in fits) you might want to imagine the mouth shape of taking a big chunk out of an apple. To bite an apple you have to raise your upper lip, frame up your incisors, and extend your jaw (just like to sing an "ah" sound). Another example, for open vowels, imagine that you are about to sneeze... that lift one does right before the sneeze? It's a good shape to enact to get in the perfect position for some vowels while singing - this is NOT true all the time but it some instances it can really help your sound and resonance.
Be aware of how much you are lifting up your eyebrows and cheek muscles
Are you manipulating the soft palate and inner nose muscles to place your sound in a more resonating spot in your sinuses? Where is your sound resonating and where do you want it to be resonating? Is your sound getting trapped or muffled anywhere?
Are you using the expansion of the rib muscles on your back enough?
Is the orientation of your spine and neck helping or hurting you?
What are your collar bones doing? How is your posture?
How loose are your knees? Do you feel grounded?
Are you dancing while singing yet? Don't neglect dancing (even just arm movement or swaying) while practicing.
Got swagger? Own your power. Relish in your unique ability to communicate truth, beauty and love. Own it, you’re the boss of your music! Check to make sure you've got swagger!
Now let's talk about emotional awareness:
While practicing, you should be on the lookout for how your emotions might be affecting your body, or how your body might be affecting your emotions. Positive feedback loops might even be occurring! Like, you came into practice feeling sad, and then your body wasn't up for activating any facial musicals to sing well because you are sad, and then all of a sudden you're feeling even more sad because your singing isn't going as you want it to; positive feedback cycles are horrible!
Your emotions can alter the way your muscles function. Muscles might not even be obedient if you're emotionally unsettled or just mentally exhausted. Getting enough sleep will affect your body and emotions - make sleep a priority! That way at least you have a chance at dealing appropriately with negative emotions. Sometimes, like when one is emotionally nervous, muscles will be spastic and no matter how hard ya try, they just ain't gonna do what you tell them to!
The way your heart is able to pump blood and transfer oxygen, the way your body decides what hormones to release, the way your singing is able to sound convincingly joyous or devastated - all of these are bodily results that can be manipulated just by whatever emotion you're feeling!

The illustration above is NOT scientifically accurate. It's from a semi-scientific health group called "Global Health Lab" but it's worth looking at for a reminder of all the different types of emotions or mental states that can be associated to certain hormones that really do result in changes to how our bodies physically react to the world.
The brain/body connection is an important reason why it is a good idea to practice positive self talk while practicing. Your body will work and listen to your brain's orders if you are HAPPY. Almost everything in life works better when your mental state is calm and cheery.
Put a smile on! Fake it until you make it - you might be surprised by how quickly your smile and "happy" emotions become legitimate.
Musicians are often required to tell a story through their music, essentially, to be actors. For the sake of good music, it's important to be practice changing your mood when you need to. If you can be devoted to music, you can devote yourself to training your emotional disposition.
How do you train your emotional disposition? During vocal practice, try putting yourself in other people's "emotional shoes", switch to communicating different emotions; be aware of your brain/body connection. Did you know that people can hear facial expressions over the phone? Smiles can be heard. Test it out with someone, go see for yourself. See if someone can hear you smiling while singing or frowning while singing. Playing emotional acting games with yourself is another experiment to try while practicing - pay attention to how your emotions and adrenaline connect to your body while singing. If you think your singing is lacking energy, try imagining your crush or your favorite celebrity walking through the door... did the adrenaline rush change your singing? If your singing is lacking passion, try imagining that your least favorite politician just entered the room and this is your chance to tell them off! Play with your emotions and see how it can impact your voice.

So how should voice students practice? Practice with imagination and experimentation.
And like those sensor lights that go off when there is a problem in your car - pay attention to what your body is telling you, listen to the tiny light bulbs in your brain.