There are several ways to do it, most have been mentioned above.
The cheap/easy/good-enough way os to mount a section of rail to the handguards. If all you want to do is mount a flashlight, and you have a good idea of where you want it, this is a PERFECTLY good solution.
The expensive/harder/best way to do it is to replace the handguards with a rail system. If you want lots of mounting flexibility, want to keep your options open as far as adding more stuff, and don't mind dropping some coin, this is the way to go. However, you need to consider how much more stuff you realistically want to add, beyond a light. If the answer is "nothing", it's overkill.
Of course, you can always undo what you do this time around, so it's not like you're making a decision for life, here. Stuff comes off as easily as it goes on.
I have a fair amount of experience with the bayonet and front sight base mounts, as we're required to use them on our rifles at work. They're acceptable, but the light is mounted out at the end of the rifle. This can create balance issues (probably not huge ones, since this light is pretty low weight), and activation issues (you need to put your hand forward, or run a tapeswitch), but if it works for you, that's a cheap and easy solution. I prefer the bayo log mount over the FSB mount...the FSB mount has the light obstructing your field of view too much for my tastes. Some people think putting the light under the barrel casts a shadow in the wrong place, but I have yet to have a problem seeing things, and I work at night. They also make a mount that puts the light on top of the barrel in front of the FSB, if that concerns you.
In short, don't stress about it. Try something and see how you like it. If it doesn't work, buy something else. You can always sell the part you don't like in the B/S/T forum.
Mike