Teaching writing is often about teaching grammar. If grammar comes
up anywhere in EFL, it is in the writing classroom. Most EFL students
will have some writing skills when you get them and they will often have
an idea that their writing is quite good. 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.