Well, to start off, I would suggest that you first look at other people's reviews and think about what they're missing, i.e. what would you like to see in their reviews? This will serve as the basis for what is unique about your reivew.
Then, look at the common elements behind most reviews. Most people will start off talking about packaging, accessories, build quality, design, and ergonomics before commenting about sound. They'll end with a concluding remark that boils down the essence of what they believe this product to be. While you shouldn't forgo the basics, you should make a point of trying to look for subtle things to remark about that other people might not mention.
For sound, something that helps me is the presence of a frequency spectrum visualizer when listening to music. I use the 80-band mode (the highest that still labels the frequencies at the bottom) in Foobar, which shows me what frequencies are most emphasized in the music at any one point in time, allowing me to understand whether a headphone's response is commensurate with the actual music, or whether its being exagerrated, de-emphasized, etc. Even though a spectrogram also helps, it's not necessary. I also do frequency sweeps with every earphone I buy, just to see what its limits are. Mine are self-generated, though you can easily find them online. I start with one that goes from 15-20000, just to get a feel of any major peaks and valleys, and then use specific ones, like 20-50, or 10000-20000 to find out specifically where the headphone rolls off. I also tend to have one that covers the mid-bass, mid-range, and treble to see if there's any uneveness in these specific areas. Of course, these are all subjective tests, and that's what these reviews are, really. With that out of the way, it's time to cover the presentation. Does the singer sound up front? How up front? Are you on stage with the singer, sitting in the front row, sitting five rows back, or back in the mezzanine? How about the instruments? Do they sound like they're on the same plane as the singer, behind the singer? If so, how far behind the singer? This is with every instrument you can identify, by the way. Then, with these instruments, do they sound like how you think they should sound? For example, the strike of a piano key, is it natural, too 'metallic', too slow to attack, or what? You don't necessarily need to use the right 'audiophile term' to describe what you hear, but it helps to communicate in jargon that people on these forums will generally understand well, so that link above will help. These are just a few things that you can do to better describe your experience of the sound presented with your earphones, but there are infinitely more ways as well.
Lastly, let us know how you get along with it. Is it something you listen to everyday, and just fall in love with more each time you put it on? Or are there some days, on some tracks, on which it doesn't quite impress you?
Anyways, these are just some ideas. Lastly, don't sweat it too much. I think the big reviewers here are big because they've been doing it for a long time, and have undoubtedly improved over time. Fretting too much over what to write will most likely just hamper your experience. After all, this is just a forum, and a review is just a collection of your thoughts. You're (I'm assuming) not getting paid to write a review, so it doesn't really matter. Yes, we like seeing really high quality material here, but most stuff here isn't, at least not on their own. People learn more on Head-Fi simply because there is so much information to comb through. Whenever I research on here, I read through all the reviews first, think about which person's opinion I respect most, and which person's ears I align with most. One opinion isn't normally going to make or break someone's decision making process --- well, at least not mine.