"Vienna information, G-ABCD, 2 minutes before border, request handover" "G-ABCD contact Bratislava information 124,3 Good by"
As far as I know there is no requirement to talk to anybody when crossing a border - with the obvious exception of FIR boundaries that lie in controlled airspace or abutt controlled airspace.
At least Dutch Mil, at a presentation at our club about a year ago, told us so.
So you can fly uncontrolled throughout Europe without talking to anyone.
Sometimes the flightplan is required to contain the EET to the border.
I understand that's something that's always required. EET/EBBU0030 in the remarks field should do just fine. (Means: Estimated En-route Time to the EBBU FIR boundary is 30 minutes.)
The best advice that I can give you is to book two hours of solid groundschool with an instructor as soon as you get back. During those two hours, do a mock plan&execute of an international flight. Learn where to find METARs, TAFs, NOTAMs, en-route weather, how to plan your flight keeping clear of controlled airspace, or asking for transits through controlled airspace, transponder procedures, customs/immigration procedures, submitting/opening/closing flight plans, booking in/out, the GAR form, different radio procedures and so on and so forth.