It's honestly not very hard. Dominance is another term for playing high status in all situations. Read Impro by Keith Johnstone for the basics. Some of them are:

1. High status people make slow, measured movements, no jerky or rapid ones. Part of this is the "low reactor" phenomenon where their affect does not change much (not the same as a frozen-face look) whatever the stimulus.
2. They win staring contests by looking away first, and not looking back. It is looking back that reveals low status
3. They pause longer before responding to questions or taking their turn in a conversation. If it is a noisy one, they might go aaaaaah until people shut up. Then they speak. Often with a pause first (i.e. they don't feel bad about interrupting first before they are ready to speak). Low-status people often clear their throat or start talking over the conversation

These 3 alone will fool 80% of the people, 80% of the time. You can acquire more subtle behaviors with practice that will get you to 99/99. A few acting or improv lessons will get you there faster, but even just practice in front of the mirror will do the trick. Note that this is a sufficient pattern of behavior, not necessary. There are ways to act high-status while acting the clown, or being highly expressive with movements and facial expressions. These are obviously harder to mimic. The basic high-status demeanor is easy because it is so minimalist.

If you are intent on faking it this way, keep in mind that you can only go so long acting high status without substance to back it up (i.e. saying smart things, offering better insights than others, making good decisions). If you act this way all the time with no substance, you'll start looking like a douchebag to people whom you meet often. The deeper danger is that if you do this sort of thing long enough and consistently enough, you'll fool yourself about your abilities and intelligence. External behaviors have a wicked tendency to seep in and become inner personality.

Many bad "leaders" display this poise and high-status/dominance permanently, even when they are saying or doing really stupid things. It is my belief that the behavior seeps into their skin and robs them of the self-awareness and doubt that might make them smart enough to actually censor themselves from doing/saying stupid stuff. Good leaders have qualified self-esteem rooted in deeper places: real accomplishments and a real sense of where they can be confident (or not). They have no problem switching to low-status/non-dominant behavior when it fits the circumstances.

This stuff is primarily useful for dealing with people once or twice and then never dealing with them again. Even here, it is a double-edged sword. You can piss off waitstaff etc. The price of acting high status all the time may be people spitting in your soup.

I had fun practicing these behaviors when I first learned them, but now that I am past the fun, I am mainly aware of the dangers, especially to yourself and your honest self-awareness. They might feel good, but they are not actually as useful as it might seem.

I now prefer my behavior to accurately reflect the actual confidence I feel in a situation rather than a fake confidence. Call it acting real.

Some people call it "authenticity" but I hate the term because it conflates a set of values (which I do not subscribe to -- values that include uncritical openness, honesty and trust) with a pattern of affect and behavior.

This "act real" approach avoids the dangers of dominance/status for its own sake. Literature is full of the consequences of hubris, hamartia etc.

You've been warned.