Make sure they ship where you are before you even start shopping. It’s terribly sad to find a dress you can’t live without , only to discover that they will not sell it to you.
Good on line shops show materials, measurements, different views ect.
By searching on shopping blogs you will get great hints from other addicts
Make sure what the rules are about returning items, some shops implement dead lines
Remember if you are in Europe and you buy from Victoria Secret (USA) you are going to get a tax to pay for importing when the goods arrive. So although the price might seem cheap remember this tax and shipping. All Saints (my all time favorite) offers free shipping.