Its all on the forums 
Link to the store list (horribly outdated, but has a link to comenity site on it)
and I cant find the link on how to
First, turn off pop up blocker.
Go to an online participating comenity retail store.
Add an item to cart.
Click to check out.
SIgn up and create an account or login as guest, some claim it works better with having an account. (i always do guest)
Fill out your shipping info, click to go to the next step (billing)
On the billing page when it asks for your credit card information is when you get the popup. If you see the page asking for credit info and there is no popup, you werent pre approved. Be sure to check the top of your browser (usually top right) to be sure it doesnt say Pop up blocked.
Seems victorias secret and express are the main 2 most people get first as they are the easiest.
*edit*
mispelled browser
Overstock.com is a good one, just add items to your cart and start the checkout process as a guest. You have to be opted-in to receive these offers, also allow pop-ups from your browser.
I received a 6500 store credit limit and once i loged in, i hit the luv bitton for an extra 300 lovin.
