How can you teach your students to write professionally?

You have to start from scratch.

Through writing exercises, make sure that students master each step all through the way.

Steps:

Teach parts of speech

sentence level

one-paragraph level

two-paragraph level

three-paragraph essay

four-paragraph essay

etc.

While teaching a paragraph, teach topic sentences, conjunctions and concluding sentences.

While teaching essays, teach introductions, thesis statements,  body paragraphs, coherence and conclusions.

Let them practice a lot. Hold session for correcting mistakes using a fabricated sample that includes all mistakes. Use technology, peer-editing and team work during the teaching process.

Do not forget to teach them genres and make students practice them.

Vary assignments to keep them motivated.

Make writing competitions to enrich enthusiasm and test them.