1Find good visual references.
This seems like an obvious step one, but I have created a few 'caps from memory and they would have been better if I had used a visual aid. Case in point: the hole layout on the ice cream sandwiches is just wrong. They are supposed to be staggered, like bricks and bad keyboards.
2Decide how you will build the keycap.
Will this be a miniature that uses a keycap for a throne? Or will the design encompass the entire keycap (like the burger)? If you're going to use the whole keycap, make sure the walls are thin enough that it will still go up and down and not rub against neighboring keycaps. If you are building for an island-type Escape, they just have to be thin enough to actuate.