How to improve students' writing skills step by step

As an English teacher,my major job is to teach my students to have a good knowledge of listening,speaking,reading and writing.However,among the four basic skills,writing is always a disadvantage of my students.When they finish a composition,I can find all kinds of mistakes,such as the wrong spelling,the wrong struction and even the tenses.How can I better their writing skills?I have reflected it for a long time,and I make up my mind to do a series of jobs step by step.Step one:Let the students be familiar with the ba-