Some units come packaged with two or more L brackets, which you can use to properly and securely install your unit. If these brackets don't come with your unit, you can purchase some at a hardware store. Screws and L brackets with two screw holes are ideal for securing a window unit to a window frame.
The bottom guard of the air conditioner window unit is a small ridge that helps to secure the unit to the window frame. Sometimes the bottom guard helps to house the accordion panels on the sides of the unit, too. Outside of the window, some unit installations call for a piece of wood to be fastened to the ledge near the outer side of the windowsill. A good way to further secure the bottom guard of the air conditioner is to fasten two L brackets to the wood piece that's attached to the ledge, and slide the guard in between the brackets and the windowsill to prevent the unit from slipping out of the window.
A window slides down on top of the top guard of the air conditioner, which is also a thin ridge that spans across the top of the unit from one side to the other. The window sash slides either behind the top guard or into the groove of the top guard, depending on the model. At this point, the sliding window isn't aligned with the locking component of the stationary window any longer. You can install two L brackets onto the tops of both sides of the sliding window to prevent the window from being raised. If the window raises, the unit might fall out of the window, which is dangerous and will most likely break the unit. A screw can be inserted through the side of the bracket with the hole in it and into the side of the window frame. The flat part without the hole should be butted up against the top of the sliding window. A bracket secured to each corner of the sliding window should do the trick.
Some units feature installation holes for screws on the lower sides of the air conditioner frame. If a piece of wood is fastened down in front of the window and supports the bottom of the unit, you can screw one side of an L bracket into the wood and another side into the unit, which should be done on both sides of the unit. If the unit doesn't feature these holes, you can make your own with a power drill, but you must ensure that you don't drill into any important components and, by all means, unplug the unit.