How do you learn English well ? How do you speak English well ?
There's no simple or single eanswer to this- people learn in different ways and with different degrees of success. Constant practice is the single most important thing in my opinion. Try to be clear about your goals- what do you mean by 'well'- well enough for what? Learning enough to work in an office where you have to use it every day requires more than learning it for occasional use. Studying in English requires very advanced writing skills, as well as others, which might not be necessary if you need to be able to speak to clients, so what's your aim?