Customs: either at LFAC Calais or EBKT Kortrijk. LFAT Le Touquet is possible too but is out of the way, and is often infested with UK PPL'ers on their first trip abroad... (duck)
If you choose Calais (or Le Touquet), you will have to file a flight plan from there to EBZH too, but the chap at the pilot's desk will do it with a smile. Remember that LFAC Calais turned non-controlled not so long ago. EBKT was never controlled, though some of their tower staff like to behave like that just the same. If you want a bigger, more "official" airport to pass customs, you can consider EBOS, EBAS, LFQQ but the landing fee will be considerably higher.
Route: it all depends on the type of craft & equipment. If transponder equipped, remain above 4500' so as to be in controlled airspace, and route through navaids (KOK? MAK? BUN? FLO?). This is by far the easiest and the most safe. If not, you'll have to remain lower, and find your way between various CTR's and TMA's. I'll be glad to give specific advice.
EBZH: kindly respect the given procedures, the field has its club of NIMBY's like most fields today and we must not annoy them needlessly. Some good info at
http://fed-ulm.be/db/ulip/ulip_view.asp?editid1=37, click on one the ULIP links to get something like an aerodrome info sheet. Officially it is PPR but on a Saturday with good flying weather the requirement is not that stringent.
If you cannot bring cash euro's, make sure to have various kinds of plastic money. In France a credit card like Visa does best, EBZH will only accept debet cards like Maestro.
Last but not least: welcome to my home field! If you choose this particular destination you must have some local friends or relatives expecting you, but if you require an aviator's assistance do feel free to pm me!
What type/kind/category of plane will you be flying?