Step1
On the Internet, you don't know what the chair feels like. You don't know how comfortable it actually is! That can't be avoided. But you can get some information to help you discern that.
Description: The product should have some sort of description, indicating what applications it might be useful for, what type of people the website is targeting. The description should also indicate the type of leather in the chair, which will dramatically affect your experience (if you're curious, see my article on the common leather types here). If there is no description, that's risky because you're essentially buying a chair off of some numbers (dimensions). Have you ever bought anything, much less a leather office chair, like that? Look for a description, it'll help you make your decision.
Pictures: One picture is great, but how about multiple views of the chair, or pictures of every color the chair is available in? Now we're starting to get somewhere!
Dimensions: How much room is the chair going to take up? If you only have 2 feet of space behind your desk, you don't want a leather office chair that will take up 3 feet. Surprisingly, an ill-fitting chair is a frequent reason for returning, so save yourself the hassle and check this first.
Step2
Before you purchase a leather office chair from a website, look at their return policy, because there is always a chance you won't like the chair you get, or it won't fit, or it's broken, etc. Here's an example I pulled up after a few minutes of searching (website address suppressed):
"All returns are subject to our standard return policy that specifies that you will be charged both the original outbound freight charge (which was included in your price) and the return freight charge. You are responsible for making the return freight arrangements with the appropriate carrier and for sending the product to us in Providence freight prepaid. We will not accept collect freight shipments. Returns of more than 6 chairs will be subject to a 25% handling fee. Chairs must be in re-salable condition and in the original factory packaging. You must take the time to ensure that the product is properly repacked so that it travels back to us safely."
So if you want to return under this policy, you have to pay both return and original shipping? Their customers are lucky this site doesn't charge a restocking fee. Oh wait, they do, if you placed a large order with them! Return policies like this one remind me of those pharmaceutical ads on TV: while some woman is dancing on a beach a man is speaking rapidly "May cause indigestion, diarrhea, hemorrhoids, sever laughing fits, and possible death." You should expect to only pay return freight, and that they accept the return unconditionally. In other words, if the chair is uncomfortable, if the color wasn't what you expected, or if you just don't like your new chair, you should be able to return it. Note: sites offering "free" shipping may want you to pay for original freight as well if you return it, so watch out for that before you order.
Step3
Who doesn't like free shipping? These companies must be pretty special to procure free shipping for you, right? Wrong. Shipping always costs something, whether you pay for it or the business pays for it. And I guarantee you the business won't end up paying for it, because they don't want to hurt their profits. So that means you end up paying the shipping, in the form of higher item prices. While this isn't always the case, if you go out and do your research, you will find that most times, it is!A second thing to watch for is product eligibility. Some sites offer free shipping only on certain items, or only once you've reached a certain dollar amount. So, if you remember anything from this article, remember this: don't assume a website that offers "free" shipping will give you the best deal; do your research! Chances are, the overall amount you end up paying for the item and shipping will be the same as the same item at a "free" shipping site, maybe even less!
Step4
One thing I want to stress while shopping for a leather office chair is the importance of research. Find out if the website truly has the best price. Find out what other people think about the product. One easy way to do this is to take the SKU number from a product you're interested in, and typing it into Nextag (http://www.nextag.com/). You can quickly see the prices at various stores, and any user reviews those stores might have on the product.Unfortunately, you can't access this valuable information if the website owner places the wrong SKU number on the page, either accidentally or on purpose. Such sites are not confident in the value they can deliver to you, so don't waste your time.
Step5
Finally, while this won't help you purchase a leather office chair, per se, it will help you gain a feel for the website that you're considering. Does the website have a community aspect to it? This can take on many forms: forums, articles, links to other web pages, user reviews on products, etc. If a website just has products for sale, what do you think they're interested in? That's right, just your money. They probably don't care about giving back to the community, or you frankly, because if they did, don't you think they would try to involve you? You should expect that a website you are going to spend your hard earned dollars with will care about you! There's plenty of websites do: they build communities, or publish articles to aid your shopping experience, so go find those.
Novice 
