I don't use instagram to promote my CPA, but I've seen on instagram this method a few times and seems to work for most of those guys.
1) Mass comment to promote your CPA offer, for example: "WOW.. I just got this iPhone 5 for free, get your own iPhone 5 for free @freeiphone5
2) On your main account (in this example @freeiphone5) put images about your offer and a bit.ly link (or other shortening url) to your CPA offer.
Good luck and keep us updated how it goes!