You have received a lot of really good specific advice so far. Just from a general standpoint, my advice is to
always know the reason why you are initiating scissors. I've seen a lot of clueless pilots just start scissoring on me without a specific reason in mind, other than they "think" it's going to throw me off somehow. All they are doing is bleeding E and making my shot easier. Your scissors manuever should always be a direct extension of how you are either going to
1) spoil a shot,
2) or neutralize an enemy E advantage,
3) or manipulate angles to get a brief snapshot.
One of those three. Maybe there's another that I'm not yet advanced enough to be privy to, but start with those three.
Often, when you are doing one of the above, you won't have to think about when to initiate scissors, because it will come naturally as a defensive-------->offensive manuever.
I was going to explain a bit about the importance of watching your bandit in initiating scissors, but Agent360 covered it already. Read below, he's absolutely right.
1. you have to learn to fly looking backwards. While looking back watch for a lag behind your tail. This means it looks like he is falling down if your going up. At the moment he goes to lag pursuit REVERSE. As he crosses your tail IMMEDIATLY reverse....you are now in scissors.