I found that a heat gun and an assortment of small chissels and a utilitly knife worked well. Heat behind the area you want to dent because the sintra will change in texture if too hot. When the sintra is soft enough then you can go to work and push in and carve out the dent. I've added a few pics for examples. If your not trying to achieve a carved look then just use a blunt shape to push in the dent.
Im working on my armor right now, and I've used a screw drive to "trace" my battle scar, then I push and twist the screw driver and get the results I want. I do it in front of my TV, but find myself paying very little attention to the program.
Yeah, wait til you get married. Saying "my wife" is even more of and interesting transition (in a good way of course). You just get used to saying fiance' and then you need to say wife. Nice armor by the way!