What comes to mind when you think of making a good first impression? Do you start with a word one description of an impression? We often begin with abstract thinking: People who make a good impression are: attractive, confident, welcoming, etc. Then we might begin to tailor the word to a particular situation. The first impression we want to make at a party might be different than the first impression we want to make on our child's teachers.
It's hard to live up to a one word description. It's easier to step into a connection with someone we know who has made a great impression on us. When we think of situations we will encounter, we could begin by thinking, "who do I know who would be great at this?" As you think of that person and the kind of impression they make, it's possible you might start changing some of your attitude, your expressions, your movements to be more like them. It's possible that just paying attention to a person who made a great impression makes it more likely that you will make a similar impression.
Perhaps you are overwhelmed by this advice. There are dozens of reasons why you might find that the only people you are sure make a great first impression are so different from you (or in a situation so different from yours) that you can't possibly benefit from the model they provide. You just can't be tall enough, confident enough, loud enough, or discreet enough to do what they do to make the impression they make. Are you just stuck with making whatever impression you make?
Maybe in the situation you are considering, you are someone who needs to make a first impression by the quality of the attention you pay to the people you are meeting. People find it easier to connect with people who want to connect with them, and easier to be fully engaged with someone who is fully engaged in interacting with them. Your best bet might be to be yourself and to simply open all your circuits to paying attention to the person you want to impress. If you're not distracted by self-consciousness, ego, or the places you would rather be, you will give the impression of someone who is fully present and attentive. That's unusual enough to make a strong impression.
Your job is not to create an impression. It's to be present enough in what you're doing so that just being your best self leaves an impression.