If I would advise against it, I am basically saying that my advice to you is that you do not do whatever we were discussing or referring to in our conversation. http://www.grammar-monster.com/easily_confused/advice_advise.htm

I'd advise against it = I would advise against it it is the "subjunctive" the whole meaning is "If someone asked me whether they should do this thing or not, I would advise them not to do it." or "I advise you not to do this thing."