What follows is an illustration of the general "decompression" surgical procedure designed to treat a Chiari I malformation. Even though this procedure has been widely adopted as the optimal form of treatment, many neurosurgeons still disagree on some of the specifics of this procedure. There is no single right way to perform this procedure.

First, the patient is placed in the "prone" position, which simply means on your stomach. Don't worry, you are fast asleep by this point!

Next, the surgeon will cut through your neck muscles until your skull and vertebrae are exposed. As you can see, an arc approximately two centimeters (wide) by one centimeter (high) is removed from the base of your skull and the back of your top vertebrae is removed. This is done in an attempt to create room for your herniated brain to reside. It is hoped that it will "fall back" into this newly created space and that it will no longer put pressure on the brainstem. This should result in the normal flow of your cerebrospinal fluid having been restored..

Once the appropriate bone has been removed, the dura layer will be opened to allow access to the brain matter. The dura layer is a transparent, skin-like layer that surrounds the brain and spinal cord. Your spinal fluid flows between the dura and the brain and spinal cord, keeping the brain and spinal cord "buoyant." Surgeons will often-times have to open the dura layer in order to remove scar tissue, or to perform other additional steps such as cauterizing and shrinking the herniated tonsils so they do not obstruct CSF flow. Remember, the goal of the surgery is to: 1) create more room for your brain, and 2) restore normal flow of the spinal fluid in this region and sometimes the dura needs to be opened in order to adequately accomplish this goal.

Finally, once proper flow of the spinal fluid has been established, the surgeon needs to sew a patch to cover the dura layer that has just been opened. This patch is sewn in “watertight” so CSF does not leak out. This patch is also known as a "dural graft" and can be made with different materials. For instance surgeons will use gore-tex, bovine material, or even your own pericranium for the “dural graft”. 

Once the graft has been successfully installed, the surgeon will sew your skin back together and you will soon wake up!

After-words, you can consider yourself "decompressed."  Sounds kinda cool, eh?

FYI, this procedure is also known as a "Posterior-fossa decompression".