1. MEASURE TWICE, CUT ONCE!!!
- If you look at the images on the previous page, the shape is first measured out, not just cut willy nilly. So I know where I'm going before I even start doing the mod.
2. Razor blade and Needle files.
- Carve away the material that is not needed, while keeping in mind that you have to follow the existing taper from the original detail.
3. How do I not break through?
- I do break through sometimes (not on purpose of course) If you look at the previous image where I did the mod under the chin, you can see that there is most certainly a breakthrough in the helmet. But to avoid this, in advance I make sure that the backing of the area will be thick enough to support the work that I will be doing.
Hope this answers your question, If you have anymore please feel free in asking
As far as how to do the round over in the front; I use a rotary tool with a 1/8" cutting bit. With regards to how I get the shape proper and smooth... See the process that I listed in the reply to Kyre.