Well it may seem obvious, but the only way to build any skill is to practice it. Each day write something in your target language. It can just be telling about what you did the day before or describing something you saw on the way home. It doesn't matter what. Then post it here or under the "Notebook" section and wait for feedback. I do that with Japanese (which I am learning). If I am lucky I get more than one response and this allows me to see the different ways that my thoughts can be expressed in my target language, yet still be correct. I study the replies very carefully and see how they differ from what I wrote - that teaches me where I may have used a wrong tense, or the wrong particles (oh, those Japanese particles!). It introduces me to new functional patterns that I may not have learnt yet, so send me off to explore new ways of expression that I can use again. Just a short piece of writing - sometimes no more than two sentences can give me a tremendous amount of valuable feedback. And I find that the lessons Iearnt doing this really stick with me much more than just reading a sentence in a grammar book that someone else wrote and that may not have much relevance to me. Give it a try!

The best method to be able to write properly is reading and practising. If you read a lot, after a while you will know what sounds correct and what doesn't. As for practice, you can do it here too - start a discussion forum or write notebook entries, which will be corrected by native speakers, thus helping you develop your skills.

The notebook function on italki is useful. You can just use all the vocabulary you already know to build sentences, and add in new stuff as you learn it. It doesn't have to make any sense, but you will be practising and having it corrected by native speakers :)

I did a free online course recently on how to draft a paragraph. It explained basic grammar, punctuation, types of phrase, types of sentence and paragraph structuring. It was very good and has given me some more skills to help people with their writing.