Low-beginners: write short sentences using the words, phrases, and grammar you know.
Mid to high-beginners: write short paragraphs on various topics using the words, phrases, and grammar you know.
Intermediate and advanced: write journal entries and essay-style responses on various topics.
No matter your level, it is crucial to regularly receive corrections and feedback on your writing from an advanced or native speaker. You may ask a private tutor or language exchange partner on an app such as HelloTalk to review your writing. Another alternative is to ask someone in the italki community: https://www.italki.com/community/questions
Lastly, when doing your writing you may not use online translators. You may only use an online or a paper dictionary to look up individual words.