Disclaimer: What follows is purely my personal opinion, based on my own experience, and may in fact be made up on the spot. Your Mileage May Vary. Take what works for you, leave the rest.

So, the thing about descriptions is that you need to understand what you're trying to do before you start. Because you're using words, you have to accept right off the bat that you're never going to be able to cram in all the details that your senses would pick up, and you shouldn't try. Visually speaking, descriptions in writing are not like photographs; they're more like impressionist paintings. The goal is not to be 100% perfect, the goal is to be good enough that your reader's brain can fill in the gaps. And they will, if you give them enough to work with.

Part of that is using the right tools for the job. You want to select words that say precisely what you want so you don't have to use lots of extra words beating around the bush; but at the same time, you don't want to worry about the precise words so much that you lose your audience. It's all well and good to know about the coruscating astral bodies in the crepuscular aubergine vault, but you'll get a lot further with your readers if you tell them about stars twinkling in the sky at dusk, because everyone knows what those words mean and they've all seen the evening sky before.

On a related note, it helps to describe things in terms of what your audience is already likely to be familiar with, and this is where metaphor and simile come in. This is pretty much the only way to describe smells, too, which tend to be the most overlooked despite being incredibly important. Again, it's one thing to try telling your readers about the pungent aroma of petrichor, but a far better thing to tell them it smells like the earth after it rains. Just about everybody can relate to that without needing to know a bunch of fancy words.

Another thing to keep in mind is that you're not just trying to put a picture or sound or smell in your readers' heads; you're trying to make them feel something. How people feel about an apple is likely to be very different from how they feel about a stop sign, so if you're looking for a way to tell us about the color of a girl's lipstick, think carefully about how you want us to feel about her. Stop-sign red is a warning; apple red is an invitation.

The right verbs are important when you're describing things in action, and this is a place where you really do need to worry about precision. To take an example from this essay about writing fight scenes, there's a big difference between hitting someone and smashing your fist into their jaw. You must also avoid passive voice like the plague. This is tricky to get a grip on, but in a nutshell, it's the difference between "Steve punched Joe" and "Joe was punched by Steve." The first sentence is active; it describes a thing (in this case a person) doing an action. The second is passive; it describes a thing having an action done to it. As a rule, active voice is more exciting than passive voice. It uses fewer words, so it's quicker to read, and it forces you to write the events in the order they actually happen, which means your readers can follow along without having to stop and update their mental picture all the time. Active voice is your friend.

... I think that's all I've got in me right now. Any specific questions I didn't cover? That's admittedly a very shallow overview.

~Neshomeh