Sure, it may not be the best decision in the world to be jumping up and down in a straight jacket with the biggest show of smiling teeth before getting that needle shot in your ass. But, hey, let's take it a step at a time.
You're prefrontal cortex is where all the executive functions are, for the most part. These are highly interconnected to numerous emotional regions of the brain- endorphins, serotonin, dopamine, anandamide, just to name a few.
I'm sure you've heard the basic argument that your thoughts do control your emotions and vice versa.
The thing with positive thinking is that if you start off and say "i am happy" and you don't feel that way, it's just a thought that doesn't help trigger emotion in most cases.
But if you start off with things you know that make you happy, it's accepted, it's common sense
example: i love it when i receive a compliment
i love it when i perform better on an assignment than expected
i love it that i've got the will-power to do some work even if I don't enjoy it
I love thinking about how positive thinking is going to push my mood up, with persistence and determination
I love how when there is great happiness, all problems seem easier to manage, and stress just flows away.
You will have to deal with thought limitation. These are thoughts that try to convey a sense of limitation, as in, "I won't be able to get that happy", "happiness won't solve all my problems", usually thought limitations are a homeostatic mechanism to keep your mood in the norm. That's not what I'm aiming for. I wouldn't even bother paying attention to them.
You just keep thinking up and up and up. When doing this try to keep your thought processes fast and confident. It also helps to imagine yourself how you feel after a good workout, or after a good song, or something like that.