Teaching writing is closely related to the teaching of grammar. Most of your ESL students will have some writing skills when you start teaching them. Unfortunately, it will very often be quite poor. In fact, they often will have quite poor writing skills even at the sentence level. Therefore, you will need to take them back to sentence level and begin to teach them very basic structure and how to write simply.
Run-on and fragmented sentences will be very common until you correct those errors. The more basic you get with your writing students, the better. Once a good foundation is built, you can move on to basic paragraph writing and on to essays. These skills take time to develop though. You may find resistance from your students when you take them back to basics. This can be done without demoralizing them or making them feel their skills are poor. Tell them that they can make the good skills they already have even better by polishing up some good basic skills.
When you are teaching, always remember to help students practice and acquire a skill and become competent with it before moving on. Slow things down and be sure to tie each lesson in with the previous lesson, linking their skills together as you go. That will facilitate long-term retention of the skills rather than the typical use it once and forget it approach many teachers take. A skilled writing teacher is a rare find. If you are patient and make sure your students pick up these skills, you will be a MASTER teacher in your own classroom and no word could explain the satisfaction upon seeing the development of your students in their writing.