The best way is to let it happen, Let nature take its course however you need to watch your filter wont suck on the fry up, Normally a breeding net would help but it does add some stress for the fish. Up to yourself really, If you wanna keep the fry then go for a breeding net.
I'd say she probably still has at least 2 weeks. Guppies gestation lasts about a month so it isn't a horribly drawn out process. As she nears her birthing date the stomach will inflate quite a bit more, you'll notice the stomach kind of flatten out near the front of the chest so it will look kind of boxy, and you may even begin to see small eyes in the abdomen considering your fish is pretty light colored