I had no idea how to do it a month ago, and now I'm pretty good at it.
To solder the led's you must first remove the old leds and solder. To do so, heat up your soldering iron for a few minutes, then touch the solder and it should liquify almost instantly. While you are still touching it, use the desoldering bulb to remove the old solder, and take out the led. This should only take a few seconds as to prevent damage to the board.
In order to apply new solder, heat the plate that the old solder was once on, and touch the solder to the hot plate and the solder should melt almost instantly. Your iron should never touch the solder directly.
I recommend practicing A LOT on old circuit boards that can be found in phones, computers, certain toys, etc.
I also recommend at least a 30 watt soldering iron and don't go to the 100 watt unless you have experience. The more watts an iron has, the hotter it will get. You don't want to get it too hot, and you don't want to have to hold the iron on the board for too long or you risk the chance of damaging it, so I recommend a 30 or 45 or 40 or whatever.