There are two main ways to attach the holster to the belts in ANH, the standard verison using 2 screws or rivets and the hero approach using 4 screws or rivets. This is most likely due to the heroes being the only ones to have their E-11 blasters holstered, when Han and Luke escort Chewie into the detention block. They both also have a piece of string holding their holster's tight against their thigh's..
Anyway onto the attachement, the standard way is to have the 2 leather stripes going behind the canvas belt and the rivets or screws are mounted along the lower side of the belt closest to the holster.
The hero way is for 4 rivets or screws, again with the 2 leather stripes going behind the canvas belt and the rivets or screws are spaced evenly with 2 along the upper adge and 2 along the lower edge. It look like Han has his mounted using screws and Luke uses rivets.
Here is a close-up of Han' 4 screws
Here is Luke's using 4 using large rivets
Here a good picture showing the string on the heroes holding the holster in place.
Here is a 'Rough guide' illustration for placement