Follow the steps:
Generate the outfit using the BS. They are *.nif files and are usually somewhere in data>meshes folder.
Open outfit studio and drag the generated meshes (usually ends with _1 or _0) to it.
Make sure you have selected meshes tab in right pane. Others are bones, partitions, colors etc. No need to touch them.
If your outfit has a body (check the right pane for base shape or UUNP etc) select it and use decrease mesh volume tool ( upper left, looks like black half eaten fruit) to decrease the volume of body. Again make sure UUNP or base shape is selecte