confirmation and a full weather and NOTAM brief, all in a few minutes. There are other sites too.
In section 18, you enter EET/LFFF hhmm, where hhmm is the number of hours and minutes after departure that you will be crossing the French FIR
The rules are quite complicated, but all the information is on the General Aviation Report (GAR),
The form has detailed instructions on what constitutes a Custom's "Designated Airport" (DA). Manchester and Southend, for example, are DAs. The rules state that you can arrive and depart from a DA without prior notice, provided a GAR is completed prior to departure and arrival. It is easiest to include your return details on the same form.
At other "General Aviation Agreement" (GAA) airfields, such as Tatenhill, you can depart without notification but 4 hours notice of arrival are required, communicated by the GAR as described above
You also need,
Weight and Balance
Passport & Money (Euro's)