Rapid sketching is one of the cheapest, easiest, and most useful skills in fashion. It’s a low-effort way to build a great collection of style details.
Not too long ago, there were no digital cameras. Buyers and fashion journalists would go to runway shows at fashion market week to see what designers were presenting for that particular season. These buyers and journalists would go to as many shows they could beg, borrow, or steal their way into. Once in a show, chances are their view wasn’t the best, and even if they did get a front row seat, they’d have about 10 seconds to get a look at every outfit, which might consist of several styles. Without rapid sketching, these professionals would have had no way of sorting through all the information.
Rapid sketching is basically a scribble with a purpose. Here are the steps:
1) Find a style you want to make note of.
2) Clear your mind, take a good hard look at the style you picked and mentally make notes of everything about that style you like: neckline, length, topstiching, pockets, trim… as many things as you can think of.
3) Quickly and with as little concern for artistic skill as possible, scribble down everything you noticed.

Some rapid sketches of little girl's dresses from my notebook. I didn't label them because they are very simple.
4) Label everything you just drew: draw arrows and write all the details you were trying to draw. This is the most important step when you’re first learning this technique. If you don’t do this while the style is fresh in your mind, you will forget and your sketch will just look like a scribble to you until you learn to recognize your own sketching.
The obvious question is, why do you need to do this now that we have camera phones? Can’t you just take a picture? Yes, and this is a great thing to do, but it has two drawbacks:
1) If you see a really inspirational style someone’s wearing just walking by you on the street, you might not get a chance to take a picture of that person. They could be catching a train, or just not be very open to having their picture taken.
2) If you are shopping a store and you see an inspirational style or are analyzing the brand to develop a line, you can get thrown out for taking pictures inside a store. It can look suspiciously like you are casing the store to rob it later.
Rapid sketching avoids these problems, and sharpens your observational skills at the same time.