Taking a course is helpful but it isn't the only way. Find someone who is better at writing than you are and ask them to proof read your work with you. Ask them to explain any corrections where you don't see the problem. Grammar is just a bunch of stuffy rules about what happens in different lingual situations. Once you start to memorize the rules, your writing will improve. It's gotten to the point for me where I can't remember the specific rules but a sentence may not look right and I'll reword it. Another tip I use to help the flow of my writing is to read it to myself out loud. A lot of times this will help me catch more things than just looking at it over and over. I have a problem with run on sentences so I know that if I run out of breath reading one sentence, I should probably break it into two :)

Hi... If you know your grammar you just need to practice for exemple you can try to answer Italki questions...

try to start by writing down your diary so it is quiet acceptable to make mistakes ,then things would improve gradually .

Hi what sort of writing are you talking about? If it's an essay go to; http://www.italki.com/knowledge/contribution-HOW+TO+WRITE+A+BASIC++ENGLISH+ESSAY.htm

Reading a lot :)