The Brass Monkey is considered an advanced pole dance trick that requires shoulder and core strength. You should have mastered the basic invert and have moved on to start learning the Flagpole hold since this move is using the same type of grip.
Here is one video tutorial on it:
There are a couple of ways to get into the brass monkey pole position: for beginners, a recommendation is to invert, push out as if ready to do a caterpillar and then weave the outside leg in front of the pole to make the hook/grip that is used to help hold you in position.
You can also get into position from the Straight Edge hold. The video below demonstrates both:
You can also use the brass monkey pole position to climb the pole, as shown in the video here: