You do this by modifying the block reward for the first x amount of blocks to have a higher amount of coins, then do the math and work out how many you need to mine to get your desired premined amount. You'll want to spread it across more than a few blocks depending on the difficulty target, otherwise you'll mine too many of the initial blocks too quickly.

Unless you don't mind going over the premine a little. In that case, just make the first block worth 99% of what you want your premine to be. Then mine it before releasing the wallet. Simple.