Giving orders is always a tricky area. How do you give orders to a junior officer? What if they are someone who is actually higher in club rank than you are or has more experience as a role-player?
The answer is simple don't think of it as an out of character action instead place yourself in the mind of your character. They have earned the right to be in charge and be in that position. That experience gives you the right to be confident and to know your orders will be obeyed.
Think of the following Four C's
don't gibber or be confusing. Say what you need to say.
it is a military environment so it is fair to act as if your orders should be obeyed.
don't scream at another person's character unless it is actually required. It can be counterproductive
be clear as to what your orders are and what the task is you're delegating.
Finally, be respectful and polite. Try not to scream and rant in character unless it is required character development.