First point your browser to the Openskyquery simple query page (1) at http://www.openskyquery.net/Sky/SkySite/OSQForm/default.aspx
Select from the pull-down list of major catalogs (2), which is also shown in the left column. Here we choose the Rosat catalog. Then click on "Next -->"
Choose the maximum number of objects (3) and sky region that you wish to search (4). You can also select on which of the popular catalog attibutes to show, and query on these.
Click on "Update Query" (5) and read the SQL to see that it is correct.
Click on "Submit Query", to get the results. Note that you can choose alternate formats (7)
The advantage of using Openskyquery over Vizier is that you can do crossmatch of the major catalogs as well as query them independently.
