At about 10,000 M turn to 45 degrees mark and hold it there. Open the map view and watch until your target arch hits over 100k. At that point hit x to cut your engines and coast to the apex. Once there line up the directional widget (can't think of the name) and burn until on map view it becomes a perfect circle around the planet. Hit x to cut engines again.
ETA: this guy does great videos on everything about KSP LINK
This post was edited on 8/14 at 3:57 pm