I do all the masking before the Alclad. Paint all non nmf areas first, mask those off. Now work on the black undercoat. As someone mentioned above, it must be glossy and flawless.
Now, decide on the finish you want- polished aluminum, just plain aluminum, or white (weathered) aluminum. A highly polished aluminum is not normal, only found on squadron commander's or museum aircraft. Just plain aluminum is probably your best bet for starters.
Once you get experienced, you can control the sheen by the thickness you put down. I can make polished aluminum Alclad look like their regular aluminum by putting it down thick (their regular aluminum does not look that polished if you put it on thin). But I suggest you use the regular aluminum (I think at some point they called it airframe aluminum).