Something from a well known prop maker. This is how he suggest you put dents in your knee armor. It is the same idea for the rest of your armor.
"Depending on the movie version, you may or may not want dents. If you choose dents, these can be achieved through heat, and a little manipulation. I would NOT recommend using a heat gun. Too hot. Same goes for a hair dryer. Their heating areas are too wide for this application. You should use, a cigarette lighter. Yep. If you don't smoke ... go get one at your local 7-11. You'll thank me for it. You will also want something small and blunt, about the shape of the tip of your pinky finger. No. Don't even think about it. DO NOT use your pinky for this next step
Mark where you want your dents with a pencil. Use the cigarette lighter to localize the flame on the one spot you want to dent. Doing this from behind is recommended because there will be some soot and discoloration. After you've had the localized flame from the cigarette lighter for about 5 seconds, put the lighter down and use the blunt instrument you chose to lightly push the area inward that you just heated. Instant dent. I have to reiterate, that a heatgun or hair dryer is an inappropriate choice for this application. Because of the area that they can heat, the whole knee could cave in when you go to press your wittle dent in the surface "
My suggestion is to look at the Ref CD or DVD and make the dents match the suit you are wanting yours to look like. I found when making dents in my armor, I look for items around the house that could help me make the dent look the way it is in the photo.
I used this technique for putting the dents in my knees. I also used it for my Endor Commando helmet. I used a rock for my dent making device lol. I also headed from the back in case any bubbling occured it was in the back side.
a smooth rock or a medium size sharpee leaves a nice smooth dent in my knee armor. A jagged rough rock left a dent with a lot of character and edges. and smaller dents within the dent (looked good for the Endor helmet)
you could either heat up a spot with a torch / lighter and push it in with something, or cut out the area of the dent entirely and build up a dent with epoxy putty or something similar.
the latter is how i made the dent in the bradleyfett helmet i built up: