Since I had a cab to work with, I began by gutting it and taking out its internal framework....knocking out the monitor supports, etc, etc....as I was reconfiguring it.
Then, I CLEANED IT. Washed it, vaccumed it, etc.
Then, I sanded it down..but not to the bare wood, if you use a heavier oil based paint you wont need to prime it..or if you want, you can prime it if you so choose. Up to you, depends on what paint and finish you are trying to achieve.
I puttied up the holes and nicks and then sanded them down.
In the mean time, take it slow. I cut my base-boards for my new control box one day, did the top another.
here's the most important piece of advice I can give:
TAKE YOUR TIME, DO NOT RUSH THE PROJECT
Take it slow and do it right the first time. You wont have a MAME Cab to play in a day, or sometimes even a week or a month, depending on how much time you can devote.
Just take your time and do what you like, when you want to....there is no right way.