How to sew a Buckle Belt of a Trench Coat

How to sew Trench coat buckle belt (with slide adjuster swivel of main fabric). Cut the main fabric and interfacing as a square(bigger than a belt), then attach both with ironing heat in this time. As another way, cut the main and interfacing 1cm lager than the pattern (The amount of shrinkage with ironing heat), then attach both and cut according to the pattern. This is also OK. Pin buckle needs eyelet stitch or an eyelet. Especially in order to pass the pin through the edge of the belt and stitch, need