The steps are quite easy. Go to your Button module settings. Proceed to the Advaned tab and click to open the Custom CSS toggle. Then look for Main Element. That is where you need to past or write the following line of CSS:
Now you probably notice an issue. The text is not centered. This looks bad to me, so let’s go in and add a second line of CSS to solve that.
Beautiful! It doesn’t get any easier than that, does it! Now your Divi buttons are fullwidth in the column and the text is centered! I hope you enjoyed this little tip!