Good question. Tough question, too.
It gets back to how you learned to write a brief in the first place.
If you learned by doing, as I suspect most brief writers learned, the answer is, well, you practice by doing too.
But that's a lame answer. There has to be a better way to sharpen your creative brief writing skills.
Well, it just so happens that I have a great exercise. You can do it anywhere. You can do it any time of day. It doesn't take much time at all. And the more you do it, the better you'll become at writing briefs. Trust me. I do it a lot myself and it works.
I don't know if you can do it in your sleep, but it's worth a try.
Essentially, it's an exercise in writing a single-minded proposition (SMP), which is the most challenging part of the brief. Because if you can write solid, inspired SMPs, you've pretty much written the entire brief. When creatives ask, "What's the brief?" they're really asking, "What's the one thing we need to say?" So you'll end up getting a double creative brief workout.
Like any exercise you do on your own, only you will know how hard you're trying. Cheat, and you hurt only yourself.
You could begin with an existing client's project, but that's not quite as fun, and to make it fun, take control and pick something you like.
Try this guy, for example.
I'm serious. I always start with him when I get to this point in my workshop. Anything, animate or not, not only possesses, but deserves, an SMP. Yes, even your mother. Especially your mother.
But start with this familiar face. Ask yourself the same question about Mickey that you'd ask about any other brand. Because he is, after all, a brand. Kinda well known too.
Do you see where I'm going with this? You can practice writing briefs for anything you look at all day long. The chair you sit in. The account person in the cube next door. Your pet.
Everything has its essence. Its one key thing. Its single-minded proposition. Find that and you've written a creative brief.
So what did you write about Mickey?
Next time you're eating lunch or sitting in a really boring meeting, look around you and write an SMP for someone or something in the room.
Remember John Hegarty's rule that the proposition is the first ad for the creative team. So don't be lazy. This is a test.
Be pithy. Be clever. Be succinct.
Before you know it, SMPs will become second nature to you.
Because you'll be well practiced. Without ever having lifted a finger.