If she's hanging around the bottom of the tank lots on her own she is probably ready to give birth any moment.
When it comes to fry there are two options in raising them that are imediatly available to you;
a. Buy a breeding net; you can either move the mother fish into the net just before she is about to give birth(make sure the net has decent shade and flow from the filter and some peices of plant in it, preferably ones that have lots of little leaves for the fry to hide in when they are born) or simply collect the fry up after they have been born and put them in the net.
A breeding net can hold up to 15 to 20 fry, anymore and you need to get another one; breeding nets are a good way to ensure good survival of the fry.
b. Make sure the tank has lots of tightly planted areas or marble/rock piles for the fry to hide in when they are born; this way has mixed results as fry can still be eaten by the adults but is good if you are not too bothered or don't want huge amounts of fry to survive.
There isn't much you can do to make her give birth faster, just make sure she has a stress free enviroment as much as posible(nice planted areas in the tank and hiding places; she won't want to be disturbed by other fish when giving birth) and let nature take its course; making sure she has a varied nutitious diet will help ensure healthy fry and make it easier for the mother guppy to recover after birth

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ps: if you want to breed guppys or other fish more in the future it will probably be worth buying a fry tank in the future as it makes the whole process alot easier for you and the fish although is not that nesarsary.