how do you design a basement bathroom there is a8x8 foot area but no rough in plumming
there is a sewer pipe at edge of area to be used

Your question is a little too broad to be covered simply here. I suggest a vist to the local library first. You also need to know a few things about your new bathroom before you start thinking about the rough in. To know where to rough in the plumbing you need to know where the fixtures are going to be first. Also if they will be on a sub-floor or not. Keep in mind that you will need to run a vent stack up to the roof to service all this new plumbing.

Check with local building codes. A vent stack to the roof may not be necessary in your area.
It depends a lot on how close to the original stack you are locating your bath. If your can't design within say three(3) feet, you may need to consider a plan that will allow you easy access to a straight shot from the basement to the attic.

Normally in basement remodels we try to encourage the client to build around the existing stack. This is going to be a 4" cast iron or PVC pipe. If this stack runs to the roof, which it should. You can tie into this pipe below the floor line, and with a drain from your shower and vanity. Naturally your toilet and shower/bath drain will be below floor level but your vanity can run slightly down hill and tie into the stack more or less horizontal.

Peace
Neil