It depends what (and how) you're trying to print.
If your print driver program is executable, it should be possible to debug the sapscript form by activating the debugger in SE71 first.
If you're trying to debug the reprint of something like a billing document, activate the debugger via SE71, then launch VF02
Goto > Header > Output
Select an entire row that's already been processed successfully (Green square icon).
Repeat Output
You should see a new entry near the top of your list with a Yellow triangle icon (Not processed).
Highlight that row and make a note of the Output Type (e.g. RD00) and the Transmission Medium (e.g. External Send).
Set the 'Dispatch time' to "Send with periodically scheduled job". Hit 'Enter' then 'Back' then 'Save'.
You should be returned to the VF02 selection screen with a status bar message telling you your document has been saved.
In the same session, navigate to the ABAP Editor via transaction [ /nSE38 ] or [ /nSE80 ].
Execute program RSNAST00.
The selection screen options in this case will be
Output Application = V3 - Billing (because the document we've just reprinted was a billing document, accessed via tcode VF02).
The application may be different in your case if you had tried reprinting a Sales or Shipping document instead.
Object Key = blank.
Output Type and Transmission Medium are the values you took note of earlier (i.e. 'RD00' and 'External Send' respectively).
Execute the program (F8), and you should now see the initial popup screen of the Sapscript Form Debugger.