its an awkward thing to learn at first, but don't worry, just keep at it. Don't measure yourself by what others can/can't do since everyone's prior level of experience is different. Even if you keep stalling it, just keep working away at it. Easiest way is to hold revs constant (a little more than idle), and slowly release the clutch until it starts to grab, then continue to slowly release the clutch. If it stalls, you released the clutch too fast or you did not have enough rev's. The 250's used for most courses, you don't even have to use any throttle and they will go without stalling, if your slow enough releasing the clutch. Just know that a slow clutch release is hard on the clutch, but ok your just learning.
Downshifting is a little tricker. Being a smooth downshifter takes time to learn how to rev match/throttle blip takes time (when your not coming to a stop, although i recommend blipping throttle through each gear as you slow to a stop, you never know when you need to get out of a trouble situation and need to get on the gas again. You don't want to find out you missed your rev match, let out your clutch with the rev's too high, and the rear locks up on you unexpectedly the moment you need to get on the gas. Safety first. )
Just remember, its not about being quick, its all about being smooth.