Assuming that the aircraft is amateur built and currently registered in Europe then it should be fairly straight-forward. Have a look at
this document from the LAA which may not be fully up to date, but will give you an idea.
You should be able to find the requirements in each country's AIP in section GEN 1.3, but there is considerable variation in the information provided by different countries.
I believe Spain and Portugal still require prior permission, but most others are accepting of the general ECAC agreement.