Take something you have already written and rewrite it with half as many words. Start with something that is around 1500 words and cut it down to 750.
It is difficult to cut out pieces of your writing to meet a particular word count limit, yet it is often necessary. What you learn to do as you practice the cutting aspect of editing is to clearly identify what it is you want to say.
When you first begin to cut, it seems that everything is important and you may feel anxious about taking information away from your readers. As you struggle with this, you are forced to clarify, simplify, be more direct, specific, and concise.
The first step is to identify the subject, problem and solution of a piece of work. Summarize it in one or two sentences. Next, identify statements that support each of those aspects. List them in columns headed subject; problem; and solution. Then order the supporting statements according to importance: what bits of information and ideas are absolutely necessary to make your point? What is interesting? What is complimentary, but not exactly essential? What can you let go? Order these on your list.
Reread the piece of writing and see if you can combine two or more pieces of information in one sentence. Cut out some adverbs and adjectives (but not all of them!), and cut anything not relevant to your main storyline.
Put the two pieces aside and in about a week, return to them and read the shorter version first. Make final edits to insure it accurately expresses your idea and intention. Then you can finally read the longer one, your original, to see if all that extra verbiage really makes a difference or not. Keep to your word limit and work with the shorter one until it satisfies your intentions.