"Cool, what's that look like?"
So, first I should say: there's never any question that your character is capable of opening their brain to the psychic maelstrom. The existence of the psychic maelstrom is one of the few common features of the Apocalypse World setting, and if your character hadn't been capable of dealing with it to some degree, they would have lost themselves to Ψ-harm. (The idea of Ψ-harm is introduced in the second edition, and further cites will come from the second edition text.)
Second, I should say: when you're a player in an Apocalypse Engine game, it's totally fine to talk about the names of the moves you want to make. A lot of people make the mistake of assuming the MC rules apply to everybody, but MC moves are more like plot prompts, and naming them doesn't actually help convey the plot. But player moves are scripts that both the player and MC have to agree on to some extent, and if you try to agree on what move you're making without saying its name, you're gonna have a bad time.
Here's your opener in the MC section (p. 148), discussing how players make the move:
The player will want to choose a topic, naturally. She’ll say “I open my brain
about Tum Tum” or something.
Which, as previously said, is fine to start. Everything else is going to come out in conversation, and the section goes on to detail some things the MC may want to start out by asking.
At first when you ask questions, they can be simply to establish facts and
images, questions like “what’s the psychic maelstrom like for you?” and
“how do you learn things from it?” As the game progresses, though, ask
questions about the characters’ lives, pasts, psyches, souls.
So, how do you trigger opening your brain? However you say you do, subject to MC approval.
It's probably going to wind up being something at least notionally meditative? Though maybe not even that for playbooks like the Brainer and the Hocus that are right next door to the psychic maelstrom anyway.