How should I go about a descriptive writing task?

To an extent, descriptive writing is one of those things that can't be taught. Having said that however, there are a few things that you can do to help greatly improve your writing.

One good technique is to focus on the five senses, and to base your description around them.

For instance, if the question asks you to describe a forest, it is quite effective to talk about what you see, smell, hear, see, feel and taste. For example, the squishy and yielding soil beneath your feet, the smell of mildew and dampness, the sight of soft green light filtering through interlocking branches of trees.

It can also sometimes be extremely effective if you use the senses unconventionally.
For example, "I could taste the anger of the sky." Obviously, anger can't be tasted, but this shows a high level of creativity.

Also, don't underestimate the power of similes, metaphors, personification and sound devices. 

"The tree was old, and bent with rough textured bark" is no where near as effective as "The tree leaned forward like a stooped old man with wrinkled skin trying to tell me a secret."

The two images are roughly the same, but there is a huge difference in how powerful they are.

Laslty, I would say its very important to avoid cliches. An examiner marks hundreds, if not thousands of essays, and eventually he will get sick of something being described as being "as red as blood" or "as orange as a sunset"  Try and use fresh and original imagery and comparisons. "Red as crushed strawberries" is a lot more striking, or "the orange of egg yolks."

Try and be creative. This piece of writing is one of the few areas where you have some artistic license, so take full advantage of it.