First you should select in the Celestial body box the body on which to place the vessel.
If applicable, select a spaceport in the Surface base box, and a Landing pad for your vessel.
Alternatively, you can directly input the equatorial coordinates of your landing site in the Longitude and Latitude boxes.
Use the Heading box to adjust the orientation of your vessel on the ground.
You can use the spin buttons next to the Longitude, Latitude and Heading boxes to adjust the values (small spinners scroll by small amounts, large spinners scroll by large amounts).
If you enter numerical values directly, you must press Apply to register the new values.
In the current version, docked assemblies of vessels on the ground are not supported. Therefore avoid assigning a surface position to a vessel that is part of a docked superstructure.