First you have to be an officer (duh!) 
Secondly, your drills must be zai, or at least not too f*cked
up.
Thirdly, your RSM must like you (or hate you), since he will be the
one who decides. Like you = sees it as being an honour for you;
dislikes you = sabo you
Of course there'll be training lah. Not as easy as it looks.
Usually months, esp for important parades.
