This video is very good at showing it, and helped by the fact that the chap is actually driving a 500, so you can get a feel for the speed and sound of what it can be like in a 500.
When I took my 500 for it's first drives, around this time last year, that was my first time driving a crash / non-synchro gearbox. It was a bit daunting to start with, but I muddled through, and now I don't even realise I'm doing it.
I wouldn't fit a synchro box now, even if I was gifted one. That involved nature of getting the grips with the crash box is all part of the character of the car to me now.
If it helps, when I was learning my mistake was not to give it enough of a rev on the downshifts. It looks like you should just need to tap the throttle to get the revs up, but with the heavy flywheel on a stock 500 I often find myself giving the throttle a good half throttle or more stab to really bring those revs up.
Likewise, if you let the engine RPM drop further when practicing your downshifts it can be a bit more forgiving. To downshift from 4 to 3 while you're still doing nearly 40mph, right near the III shift point on the speedo, requires a big rev match and the car will really complain if you get it wrong. But if you wait till you're doing 30mph or slightly less to do that downshift then you don't need as big a blip of throttle and I also feel with less engine speed, and less flywheel inertia that it's a bit more forgiving to ending up with a miss-match of speeds.