If you create the coin, you get to make the rules. Nothing stops you from patching the software to make your premining as easy or as hard as you want. For instance, you could set the difficulty schedule so that the first billion coins require very little or zero proof of work to mine; then you could mine them all at once and award them to yourself.

Of course, the hard part becomes convincing anyone else to use a currency that's so obviously slanted in your favor.

The rules for setting the rewards in the blocks can be as simple or as complicated as you want. Most Bitcoin derivatives have followed suit with fixed mining block rewards that are halved after some amount of time, just changing such amount and the time after which it gets halved.

Others, (as Lucky, Doge, Cat coins) have a random block reward whose mean value is reduced after some amount of blocks. Plus, some coins (42 and formerly Doge) have a random "superblock" where reward is much higher than usual.

Primecoin and its derivatives have the reward dependent on the difficulty of the network, and there are some other, weirder models.

The "premined" model is usually achieved by defining the rules so the genesis block yields an incredibly high amount of coins and the next blocks give out a more "normal" distribution with a normal decay of reward. That way the creator of the coin, just by the act of creating the genesis block, automatically becomes the owner of a significant amount of coins.

"Insta-mining" is similarly, defining the yields so the first blocks give a very high reward which then decays extremely rapidly - so creator of the coin and the very first miners (usually the creator's associates) end up with most of the coins. Quarkcoin was highly criticised because of this, in the end was more or less accepted by the community after its novel approach of multiple proofs of work was deemed interesting.