How to buy online smut manga

As someone who uses Renta as a means to legally obtain manga, having your money slowly deplete as you binge read everything has become a problem. More or less, especially with smut, you have to break the age old saying "don't judge a book by it's cover" and actually start doing it. I have made the mistake of impulse buying and as a poor, jobless, university student, savings are everything. So by making plently of mistakes, I've come up with some rules I follow to ensure I don't buy shitty smut manga.