Information on how to do it is usually impossible to get. It's not really economical for lbs to service them and manufactures often provide no technical documentation.
[edit] The above applies only to low end forks, particularly those produced by companies that don't also produce high end forks.[/edit]
If you want to service it you pretty much have to work out how it comes apart. Low end forks tend to be fairly simple. Often they just contain a couple of springs and no damper.
A quick search on google came up with this.
link
No oil. Just a spring in the cylinder on the left, and empty on the right. you can remove the cover under the adjuster, with a large adjustable wrench - carefully!