There are many, many different forms of Qigong. Writing an article with hard, fast rules to be followed in all forms would be misguided. However, there are some general guidelines you can use to start a new practice or deepen your current practice.
Step one: Settle into a full, relaxed rhythm of breath before your begin your practice. The quality of your breath is directly related to the quality of your practice. For the vast majority of people their breath is bound by habitual patterns and stored tension. Your goal is a state where the breath rolls in and out smoothly, like calm waves. No rushed exhales, no gasped inhales. This is your goal but if you force it you are just generating a new form of tension and distracting yourself from the practice.
It is beneficial to stretch the breath before you begin your practice but then forget about it while you practice. To stretch the breath start with a simple inhale for a slow count of six, hold for three, exhale for six, hold for two, repeat. Anytime you become short of breath or tense let the pattern go until comfortable and then return to the holds. As this becomes easier expand the breath to a count of eight, ten, etc. holding for half at the top of the inhale, a third at the bottom. After working the breath for ten minutes or so relax into a natural rhythm again.
Step Two: Settle the mind. Energy follows intent. If our focus is scattered, so will our energy be. Don't waste time and create tension by trying to force the mind to be silent. Instead look for silence underneath the noise of random daily thoughts. Focus your attention on watching your breath. As thoughts come up, no matter how clever or seemingly prevalent, notice that you are thinking them and turn back to the breath. Gradually the mind will become quieter. You can settle the mind as you are stretching your breath.
Step Three: Relax the body. Scan very slowly through the body, head to toe, releasing tension as you go. Repeat three times. Every part of your body should feel at ease and heavy, sinking toward the ground.
Now you are ready to practice your Qigong set. Qigong is movement and breath in unison. In general, unless specifically trained to do otherwise for certain movements or forms, you should maintain deep, relaxed, even breaths. Because the breath and movements are linked, the moves should be slow, and maintain that deeply rooted relaxation.
As far as feeling the energy goes: Maintain clarity by bringing your focus back the the form any time you notice yourself thinking. Visualizations are used for some sets but do not fixate on what you think moving Qi, or moving more Qi feels like. Once you attach to a thought your visualization has become fantasy.
Seek out a skilled teacher, someone who can offer you a full system for energy cultivation. A good sign is someone who strongly encourages your personal practice. They should also be able to offer exercises and meditations to develop the right state on being mentally and otherwise. What you want to avoid is someone who just has a form to show you, or even a few forms. Forms are just one piece of the whole and many are of limited use unless you've done the exercises to support them. Additionally, if the teacher spends too much speaking about grand theory or what the form will eventually do for you, this is a negative mark. The focus should be on the practice and where you are at now. Theory is good, but avoid those who rely too heavily on words.
You can learn more about how to develop your qigong practice here.