Well this applies to any type of model that you want to create an opening with. It's not that complicated. Take an old X-acto blade with a broken tip. Now with the back of the blade, that means the cutting edge pointing up, scrape inside the panel line that you want to open. Take your time, it's a bit long to start off, but soon enough that blade will dig in the plastic and you will see progress. Go slow and the result will be better, no need to rush things. Once the piece is removed, just finish off with a file or sandpaper to clean up the edges and you're done.
Remember, it's about having fun.
Gerry :)