I saw this on another forum once, and it looked like a whole lot of fun. All you have to do is write a memo/letter style note to whoever you want to. Like this:
Dear CEO of Petco,
Stop perpetuating the falsehoods that a living animal can live their entire lives in a space equivalent to that of the cupboard under the stairs of the Dursley household. Bettas aren't Harry Potter, nor are they magical in any way other than their addictiveness and personality. They cannot simply wave a wand and make such living conditions work. Please forward to CEOs of other major pet store franchises ASAP!
PO'd Betta Lovers everwhere
See? Just post something you want to say to anyone in memo/letter form and remember to have fun with it.