To Justify or not to justify, that is the question

Without a doubt, interior book designs done by professionals look amazing. One tip I learned is that books' interior text is justified and many self published authors don't copy this into their own formatting. This has been a problem for me. I've been  working on editing my book and when I copy and past it into the template, the words are sometimes stretched to fit the justified format. It doesn't look professional  at all. Does anyone know a quick way to fix this?