I don’t think it’s worth trying to actually intimidate someone if you just don’t have it in you to do that. Instead, what you should work on is a “take no shit” attitude.
The way this works is, if someone is doing something crappy to you, call them on it. I like to say something like, “Can you not be an asshole and just X?” where X is what you’re hoping to get out of them.
One of two things will happen. Either they’ll realize they fucked up and they’ll back off, or they’ll double down. If they double down, you have one of two options. You can either back down yourself, try to understand their point of view, or find someone else to help you out.
You don’t need to actually be intimidating, you just need to not respond the way people think you’re going to respond when they try to intimidate you.