I get the process. What I was trying to convey is how seamless and perfect the finished product turned out. If it were me doing that mod, the side pieces would be misshapen, the top notched piece would be overflowing with glue and the putty would have terrific sanding scratches, all in all the piece would look bad, much less having to make two.
Here are some tips that may help you to make modifications like this more uniform.
First, design the alteration as fully as possible before you try building any of it.
For cutting out framework pieces from styrene, it can be helpful to use the computer: draw up your design on the computer, scale it to the proper size, and print it out - then use the printout as a cutting stencil of sorts. (Glue the paper to the styrene, cut, then remove the paper.)
Start the work by building up the framework as uniformly as you can. Check its correctness regularly. Good ways to check its uniformity include taking photographs (digital or otherwise - seeing the part from a fixed perspective helps to eliminate some of your bias - and it makes it easier to invert or reorient the image, which is also good for helping you to avoid mentally "smoothing over" bits of what you see. This is why drawing things upside-down is used as a drawing excercise - if you look at things right-side up your brain interprets them, and you tend to draw your interpretation. If you look at things upside-down you don't interpret as readily, and so you'll see things you might otherwise miss.)
When the framework is on, planing down surfaces can be really easy. The easiest way, IMO, is to fill the space with polyester putty, press a styrene plate against it, let the putty cure, and then remove the plate. The result will be a smooth, flat surface that needs very little clean-up.
Both Foo's work and Bawoo's work have good examples of this kind of thing, and both are well-documented in stills. I agree that a video could be incredibly helpful (and because I'm interested in teaching techniques like these, weak as my qualifications may be, I do want to do something like that at some point) but it really is a lot of work. In the mean time, the best way you can learn to do this is by practicing it yourself. Don't hestitate to share your progress, and ask for help if you need it. Plenty of us just love to field questions like those. That kind of structural work is the core of the hobby for me.
