Command is an extremely complex topic, well outside the scope of this reply. However, when it comes to command, there are two overarching words you need to remember: intent and focus.
Intent refers to your intentions during bullpens, live BP, or games. Intention drives action. If you're trying to develop command, your intent needs to be: "I will throw this pitch to that spot." That's it. In this sense, it often helps (particularly at the beginning) to split up your mound sessions into "command" and "velocity" sessions to avoid interference between the intent to throw hard and the intent to throw with accuracy.
Focus refers to your ability to direct your unwavering attention towards the task at hand (the task often directs your intent). Psychobiological states such as fatigue or anxiety and irrelevant information in the external environment (e.g., everything else that is going on in the game) will always be pulling your attention away from the task at hand, compromising your intent.
Intent and focus are intricately related, and can be improved synergistically and rapidly in non-stressful situations. Once this is mastered, the key to excellent command is developing the ability to maintain intent and focus under pressure.
Hope this helps! I will add some more thoughts on developing command at a later date.