For this achievement you'll need to cross five different border locations between Romania and Bulgaria, and Bulgarias with Turkey.
The five crossings are:
- Slightly northeast of Edirne
- Slightly nortwest of Edirne
- Northwest of Kozloduy (nearest garage in Pleven)
- Slightly northeast of Ruse
- Just south of Mangalia and north of Varna
All crossings are marked with a purple sign showing a customs officer. For the one northwest of Kozloduy I came from the south and it already registered halfway of the long bridge, around where the actual border with Romania is, I didn't have to drive the rest of the bridge and through customs.
Once all five border crossings are discovered, the achievement should unlock.