Here's a picture of the real thing for comparison: http://www.fanpop.com/spots/john-barrowman/images/31712450/title/john-barrowman-summer-concerts-2012-photo
So I got Jack out of his coat and removed the jacket of the first Doctor. As it was a bit too long I had to shorten it to about crotch level. I also cut off Jack's weapon holster.
After putting the jacket onto Jack, I removed the row of buttons. Then I got out the Milliput and added a bit to the sleeves, so that the hem of the shirt cuff could stick out under the sleeve. Also the lapels had to be changed, they needed to be a bit longer and have the little vent near his neck:
I also removed the little white peak of t-shirt under the original blue shirt collar, and painted it in skin color. After that, it merely was a painting job, blue for the suit, black for the shirt, belt and shoes. Finally I added some glitter: