i'm not sure how it's done in real life but i managed to put a geostationary satellite above whatever Longitude line i specify with vizzy
1- Plan a circular orbit around 2986 km (calculated by vizzy)
2- Once above the desired longitude, a burn takes place to extend the apogee to 8806 km (geostationary orbit height) . The point is this apogee and perigee makes an orbital period of 0.6 sidereal day, which comes handy as explained in next point
3 - from there will make 2 passes over the apogee and on the third apogee pass, the craft will be above the same longitude and performs the circularization burn.
i tried to find any video on youtube on how those satellites are positioned but all i found is the concept of geostationary satellites orbits
PS : thanks to SFlanker for his Vizzy++ mod , it made my job easier