There could be no simpler instructions than the ones posted on DrillsAndSkills.com:
Gymnastics Parallettes – DrillsAndSkills
Need more detail?
Parallettes are a gymnastics tool that simulate parallel bars — which are similar to dip bars. You can use a pair of parallettes to do wallstands, handstands, dips, ab exercises, and tons of other weird, gymnast stunts. The following tutorial explains the process of making your own parallettes. The entire process shouldn’t take more than an afternoon, and is quite inexpensive because it uses PVC pipe.
complete instructions – Celtic Kane Online
The best tutorial on how to use them is on American Gymnast: gymnastics – parallette training – free membership required.
Gymnastics Coaching.com 