Forget the FMS to plan anything until you can fly the aircraft without it.
Start descent at 3*FL in NM. While you're still getting to grips add 10NM for slowing down. So starting at FL350, start descent at 115NM. Don't forget to correct for airport altitude, non-standard QNH and, finally, adjust for wind. Add a few miles for a strong tail wind and subtract for a tailwind.
Then, as others have mentioned, use "gates" to make sure you're on profile. For example, decide what FL you want to be at every 20NM or so.
Finally, to maintain a 3deg descent you need to descend at 5*GS. So if you start descent at 115NM from F350, with a GS of 450kts, if you fly a VS of 2250fpm you will be at F300 when 90NM out etc etc. If you continually adjust your VS to maintain 5*GS you won't be caught out by varying winds.
And when you can do all this competently without reference to the FMS, then use the FMS.