Flirting in grade school is difficult because one, you don't usually have a lot of time (exception: lunch), and two, you don't want to step on anyone's toes by flirting with someone who's taken. Word travels quickly when you're with the same group of people for years at a time.
So, I'd say that the best way to flirt would be to approach the other person with a compliment; tell them briefly and nonchalantly what you like about their appearance (that day, or in general), or what you liked about one of their school projects (if you're in the same class). It will be an ice-breaker. If you approach the guy with a compliment, the rest of the conversation will seem like flirting regardless of what you're talking about because you went out of your comfort zone to make him feel better about himself. It takes guts, but in the end, it's the easiest way.