Sketching is the main way that artists sort of build their drawing muscles and it's a good idea to just sketch constantly and to just sketch everything around you. You don't need to wait until you're sitting in a figure drawing class with a model sitting there holding up a bowl of fruit to be able to warm those drawing muscles. Basically, anywhere you are, you can always have a sketch book and if you have a few minutes, you're sitting on a train or a bus or something like that. Every artist I know has a sketch book full of subway drawings, or whatever. Hopefully, without them getting mad at you, you could just sit their and sort of capture a quick impression of how they're sitting. Maybe, the details of what they're wearing.
Really, there's no how-to about it because what you're really doing is trying to perceive things and translate your own perceptions into drawings. The only how-to about it is to just do it as much as you possibly can. You need to be an extremely confident draftsman. You need to have the ability to draw just about anything without much effort and without having to do a lot of work. And the only way that you build that ability is by just drawing all the time. So, this isn't about doing a final finished drawing. This is just about getting an idea down as quickly as you can. Just about developing the muscles that transmit an idea from your eyes to your brain and to your pencil and just getting it down fast, loose, and spontaneously and moving on to the next thing.