With a property on the web service... Pass the dataset to the webservice by setting the property to it.. then run an execute on the web service... You could, also, pass it in the constructor if you just want to send a dataset and have it proccess.
I should add- the second option is MUCH easier-- since you don't need to worry about state...
I am here to help you, not do it for you.
That is, unless you hire me. I am looking for work.