Here's a brief breakdown of how I separated each of the shoulder pads into two layered pieces. It's fairly straightforward.
The HT shoulder pads have stayed mostly the same since the original TDK suit MMS release from a few years ago, and frankly I'm tired of them reusing the same stuff over and over again, especially when it never looks like separately layered pads. The 1/4 scale TDKR figure has dual-layered shoulder plates, why can't the 1/6 DX12? So I did something about it myself.
What we're trying to achieve:
Essentially, what you're doing is cutting the shoulder pad along the lower edge of the top shoulder plate. I recommend removing the chest armor from the figure, as this makes the process a lot easier. (refer to Maglor's chest armor guides for that purpose)
I didn't take any photos prior to cutting, so I'll have to use these photos by milk mag to illustrate:
Use a sharp blade (like an Exacto Knife) and carefully cut along the edge. Warming up the shoulder pad first makes the process easier, as the knife will go through it like butter.
After you're done cutting, you have a lower shoulder plate. If you removed the chest armor from the figure, now is the time to place it back on.
Heat the lower plate with hot water or a blow dryer so that it becomes maleable.
Then wrap it around the figure's bicep, right under the top shoulder plate and squeeze it so that it takes its shape. Keep holding that position with your fingers until the plate hardens at room temperature, at which time you can quickly remove it from the arm and transfer it to the freezer to finish hardening.
You should take the opportunity to repeat this process with the top shoulder plate, squeezing it to wrap around deltoid and upper arms more closely. Place the whole figure in the freezer. It only takes a couple minutes for the shoulder pads to harden, so no worries about frost damage to the figure or anything lol.
If you have some excess roughness on the edge of the top shoulder plate after cutting, you can sand it down.
After a couple minutes in the freezer, the lower shoulder plates can be returned to the figure's arms, where you can position it to your liking.
The final step involves sticking the lower shoulder plates onto the bicep. You can glue it into place, but I used a less permanent solution: double sided tape.
Benefits:
Once finished, the shoulders will look much more like the movie suit.
Not only will they be dual-layered, but they conform a lot better to the figure's shoulders, making the shoulder pads look less bulky.
(modded left, actual suit middle, stock DX12 right)
Also, when you raise the arms upward or forward, the lower plates will shift accordingly, unlike on the stock shoulder pads that would stick up as a single piece.