It can be difficult to know when you are doing an exercise correctly. Having a trainer is probably the best place to start. Looking in the mirror is usually really bad because if you are trying to see if your back is aligned you probably have to turn your head.
A method that helps maintain form is to check your breathing. If you are breathing in a manner that maximizes air flow from your nostrils to the very bottom of your lungs you are probably maintaining proper form.
A neutral neck is one that is neither flexed forward nor extended backward. There is some curve but the windpipe is at max diameter. Flexing or extending the neck won’t necessarily crimp the windpipe like a bent hose but will reduce the diameter from the maximum diameter at neutral.
A flat back is a little more difficult to maintain- many people arch their back when they believe they are “sitting up straight.” Again there is natural curve in the spine but instead of crimping the windpipe, an arched back reduces the maximum volume of lung expansion. Most people, when “taking a deep breath” raise their chest in an effort to expand their lungs up and it works a little. But breathing down into the belly gives the lungs more room to expand (full diaphragm use, soft abdominal tissue vs. hard rib cage, shape of lungs and a few other reasons). Breathing down also helps feel the flattening of the back and rotation of the hips.
By exaggerating both inhaling and exhaling through the complete motion of an exercise a person can have a better idea if they are in proper form and identify the range to which they should conduct the exercise.
One of the many safety checks I tell people during a squat is to only squat as low as they can maintain a flat back. It is easy to look from the side and determine if an individual has a flat back- but if someone is trying to watch themselves in a mirror they have to turn out of alignment, and turning out of alignment can be worse than arching the back. If a person has a tendency to arch their back they will need proper coaching to understand how to correct their form safely.
If you are new to exercise I highly recommend talking to a trainer. Many of us do assessments for free and I usually end up teaching diaphragmatic breathing as part of the assessment.
If you have any questions please ask. I’m always open to new ideas on how to better explain and teach proper exercise techniques.