These instruments are extremely simple to design, build, and play. They can also be made to be extremely cheap (free in my case) or very expensive (if you use PVC and make a huge set with many octaves.) As an educator, I think these instruments could make for a great cross-curricular lesson involving music, physics, art, and just plain ol' fun because they are pretty simple, and have the potential to be very very cheap.
I will try to show/explain my steps in a couple different ways to accommodate for the various learning styles in the audience. If you would like to know more about the theory behind my design please check out the guide linked above.
While I will try to be as clear as possible, bear in mind that while my steps are specific to my materials, the general process will be the same regardless of what you build your instrument out of.