Assignment:
Start with determining the standards you will be using. It should be no more than three and one or two are best. Then determine what exactly the students should be understanding from the standard(s). A good thing to do is be able to state why the students need to learn the concepts - what they will be able to take away from it.
Let those things I mentioned above help you determine exactly what age appropriate content will be covered and what skills the students will be practicing/mastering. Then decide what procedures will be followed in the unit. This will consist of instruction, activities, and some forms of assessment both informal and formal - how will the teacher know the students are "getting" it?
Think about how to best introduce the material in a visual manner so the students can see the concept. Then get them involved in an activity that physically demonstrates the concept. In terms of Math decimals, fractions and percents, the paper and pencil figuring part should be the last piece.