Simple. Wait until you get bored of existing and do it. I wouldn't bring it right away, maybe wait a couple years (not millennium, simply years. I mean, it's not like there are many things to do without anything around!), but i'd eventually, after having faced and defeated my fear of death, would do it. Maybe also try to reverse-engineer the decision to re-incarnate as somebody in the universe? No idea.
An abstract situation - the Universe doesn't exist yet and it's up to you whether to bring it to existence or not. All you must do is choose one of the 3 options. No matter what you decide, your existence ends definitively right after the decision. You're just your mind and you have no family, friends, memory, etc., but you have your conscience and the knowledge about the life and the Universe. If you decide yes, the Universe will look as we know it. If you decide no, there will be nothing "forever" (abstraction). What is your decision?
'The universe' refers to everything that exists. If it didn't exist, where would 'I' be? There's no such thing as 'outside of the universe'; the question doesn't make sense.
Honestly I would bring the universe into existence simply because without something that can grow and prosper, there really wouldn't be much of a point to being around, would there?
Well if my existence is going to end, what's the point in making the decision? If the only thing to do is make a decision, and there is no time limit or pressuring factor or anything, why even chose at all? By that logic there is a fourth choice. The choices are, essentially, die for nothing, die for everything, die for another, or live indefinitely. I chose live indefinitely.
Bring it into existence.
I would be happy to see that the forms of things would change and evolve, that other beings like me could emerge.