Remember first the two different kinds of visual images you can make in your mind:
Your second step is to make associated images of the circumstances where you want to use those new powerful, bold, assertive behaviors. What would it actually look like from your own eyes?
Your third step is to run through the behaviors, fully associated. How would you feel in your body? What would your voice sound like? What would you be seeing out of your own eyes?
This will make these behaviors very real to your brain, and will tell your brain INSTANTLY to call them up when you need them, so you don't even have to think about it!