Make sure a document is open in pdfToolbox Desktop. Then click "Tools > Imposition Wizard".

You can also use the CMD+Shift+G shortcut key on macOS, or Ctrl+Shift+G on Windows.

The Imposition Wizard window that appears contains a lot of functionality.

1. The Imposition Wizard offers a lot of settings, they are split up into three different areas:
   
   Output sheet: contains settings defining the size of the generated imposed sheets, margins used, position of the imposed pages on the sheet...
   
   Imposition method: allows selecting different imposition methods. Each supported imposition methods has a different set of supported parameters...
   
   Marks: contains settings defining which marks are going to be added to the imposed sheets.
   
2. In this general preferences area, you can select the measurement unit you want to use, and whether or not you want to see previews of the PDF file currently open in pdfToolbox Desktop. If "Previews" is unchecked, page numbers are shown in the preview area.
3. The preview area shows how your imposition is going to look like based on the parameters you have set. Changes to the parameters in order parts of the dialog window are reflected immediately.
4. The "Save as action" button creates an "Impose" action using all parameters as currently set. You can use this action as-is, or you can continue to edit the JavaScript imposition configuration used by it.
5. The "Apply" button closes the Imposition Wizard, and applies the imposition configuration on the currently open PDF file in pdfToolbox Desktop.
6. Use the "Test" button to perform the imposition in the Test editor.
7. Click the "Close" button to dismiss the Imposition Wizard dialog window. The active PDF file is not modified, and no Impose action is saved.