It is not simple. It can require a lot of time and effort, depending on how you do your design and what old rubber dome keyboards you manage to find.
If your custom keyboard is a simple square (like a Poker), or a novel ergonomic design, then maybe it would be better just to build it from scratch.
In most rubber-dome keyboards, the barrel plate is integrated into the top half of the case, so you will have to spend some time cutting it off. Fortunately, it is usually at about the same height as a switch plate would be.
One big issue is that you would also have to find a way to mount your switch plate and PCB inside your keyboard.
You would have to do a lot of work in plastics, and you would need proper tools and materials. I would recommend a hacksaw (rough cutting), razor saw (fine cutting), scoring knife, metal ruler (for scoring), flat file, set of needle files (not rasps), pliers (mostly for snapping), sandpaper, acetone, spot putty and plastic cement.
It is also likely that you would need to paint the case. There are various threads about "plastic goo" (plastic + acetone), painting etc. Search the web for the "score and snap" method of cutting plastics. I highly recommend a proper scoring knife over any other knife -- they are easier to use for this and therefore safer.
You can find some of these supplies in better stores that sell plastic model kits.
