Plan out your lesson as you would normally do for the whole class. This will allow you to decide on the content and method of delivery for your students, which will assist you when you begin to differentiate the lesson.
Assess your students before beginning the lesson. This can be dine through a pre-test to determine what skill level each student is, or an interest inventory to find out the particular likes and dislikes of a student. Please note that this does not mean you will have to differentiate the lesson for 20 to 30 different skill levels. It is simply a tool to figure out how your lesson will be designed.
Decide what part of the lesson would be best for individual or grouped work. Will it be the actual content that you want to teach your student? Or perhaps it the process you will use to deliver the lesson, or the project that the students will create after learning the content? These are the kinds of questions that need to be asked in order to come up with activities for your lesson.
Group your students after you have decided which part of the lesson you will differentiate. Students can either be grouped according to readiness (based on ability), interest, or learning style. This information can be derived from the pre-assessments that were given in the previous steps. If you are basing your groups on readiness, you must decide if you plan to group based on below, or on above grade level.
Give your students level-appropriate work within their group. This does not mean that the lesson is being watered down for some students. The only difference is based on the method of delivery, but the goal and objective is the same for all students.