The first thing you do to your jacket is remove the lapels and collar all in one piece. Then cut you jacket off under the armpits. Sew enough little seams to make it fit you.Trim all the edges with bias tape or something else and reattach the collar.
Steampunk Black Steel Boned corset Belt with Real Brass Lion door knocker
26" waist Steampunk UnderBust Explorer Corset Veggie Leather 'Duellist'. $235.00, via Etsy.