Pick a route that includes IFR waypoints close to departure and arrival. Pick an altitude ideally in line with the semicircular rule or 3000 ft or below
File I
An example could be Gloucester to Alderney
File I
Altitude 5000
Route EGBJ DCT WOTAN DCT ORTAC DCT EGJA
That's it.
On departure you'll get an IFR clearance from ATC, follow it until you reach the clearance limit OCAS then do what you like but tell ATC first if under a traffic service.