Turkish trousers history
You can turn over the top, stitch it and make a place inside for the belt (the belt should not be fixed to the shalvar). One thing I noticed in my last visit to the Topkapi Palace, some shalvars have a strip attached at the bottom to go under the foot. I assume this was the Shalvar to prevent from sliding upwards when riding.
Alright this is me wearing my shalvar, compared to the miniatures it looks similar:
Some more museum pieces to give you an idea: