To make pants for any size or shape of doll you can begin by making a basic pants sloper fitted to your doll, then use that to draft patterns for all styles of removable doll clothes pants and shorts. It is not difficult to create a custom sloper pattern. To make it you will need sharp scissors, a pencil, sticky tape, and a bit of non woven lightweight interfacing. Interfacing is used as it curves well and does not fray on the edges.
When you make the sloper for your doll, you will have the basic outlines of the doll's shape in a flat form. In order to use the sloper to make patterns you must keep in mind how much movement your doll has, in order to use the sloper for a pants pattern for pants which are easily to fit onto the doll. If the doll has large non moving feet, or a large difference between the waist and hip measurement, you will need to take that into account when you draw up pants patterns.
It is easiest to make an accurate pants sloper for a doll if you have made a basic torso sloper first to learn some of the techniques. To make full sets of dolls clothes you will need a torso and a pant sloper for your doll.