Open a window or door. (Don't forget to do this first so that when the spider is ready to be put outside, you won't need a third hand to open a door or window.)
Slip a piece of cardboard under the spider, and as soon as the spider is safely on top of the cardboard, place a drinking glass on top of her. (Glass is preferable so it's clear that the spider is under the cup.)
Hold the glass cup tightly on the cardboard while walking to the door/window to drop the spider out.
If a piece of cardboard is not nearby, try this: Grab a tissue, towel or some kind of material. There's usually something nearby. Scoop up the spider (don't squish it) and drop it out the door or window.
The advantage of this system is that it is not necessary to break visual contact with the spider while searching for the tools to remove her. Watching the spider continuously may prevent her from disappearing while you locate a cardboard and glass.
In addition, the spider can be contained in the towel with one hand while the second hand is opening a door or window.
Use a towel for the spider to climb on if she's in a hard-to-reach place like in a corner or a bathtub. Place the towel in front of the spider, and she will instinctively climb on it. Once she's there, fold the towel over so that she doesn't fall off and then flip her out the window.