I have never done it myself, but I believe you should be able to find the relevant information on Help:Custom_edit_buttons.
Thank you! This is exactly what I needed.
I know it isn't actually relevant to your question, but it does have to do with your example. Interestingly enough, I have found that my Internet Explorer actually autocorrects "pokemon" for me. Not so for Chrome. I am not sure if that is due to something on my end or not but I was just reminded of that because of your example.
Hmm, that's quite strange. Not quite sure why it does that.