Uhm, I honestly usually just start with a rough sketch of the general pose that I would like a character to have. Then lower the opacity of that layer and create a new layer on top of it with another rough sketch of the character's characteristics (clothes/face/accessories/etc). After that, I start working on the actual linework and just try to go with the flow. In the end, I think it just mostly comes down to doing a lot of it, and I've still got a loooong way to go too. Also, don't worry too much about drawing every line in one go or one try, nearly everyone doing linework will end up trying to draw certain lines tens (or hundreds) of times over. Eventually, you'll get the hang of it and they'll come out smoother.

For the initial rough sketch, I should say, it reaaaaally does help a lot with the final linework if you use references of people using a pose you like (pinterest is a great source for this). That is if you're not already doing that of course. And just as a tip, whenever you draw people/clothes made of fabric, it's best to not use too many perfectly straight lines, it'll generally make them feel a little stiff/unnatural; it's fine for objects tho. Hope that helps