To install QuickLook Plugins, you simply have to put the .qlgenerator file in one of two places. To make the plugin only available to the current user, put the file in the ~/Library/QuickLook directory (~ being short-hand for your home directory). To make the plugin available to all users, put the file in the /Library/QuickLook directory of your hard drive (this requires administrator access). In either case, if the QuickLook folder does not already exist, you will have to create it.
Please note that QuickLook is a feature introduced with Mac OS X 10.5 Leopard, and therefore these plugins will not work on any other operating system.
The plugin is installed – what now?
Simply select a file in the Finder or in spotlight and press the space-bar.
The plugin in installed, but it does not seem to be working
To make previews show up quickly, OS X automatically caches the list of active plugins. This means that the new plugin will eventually be picked up, but it may take an indeterminate amount of time. To force things along, there are a couple of options. If you're comfortable with the command line, execute either the qlmanage -r or the qlmanage -m command. If that doesn't work, you can try restarting the Finder. To do this, hold down both the ctrl- and option- keys, click the Finder icon in the Dock, and select "Relaunch". And lastly, some users have reported having to restart their computer before the plugins become active.
Using QuickLook outside of the Finder
In addition to the Finder and Spotlight, QuickLook can be accessed from several other applications.
In Mail.app, there is a QuickLook button next to the Save button at the top of any e-mails with attachments. Less obvious, however, is that if you select an attachment in the message itself and press the space-bar, it will show a QuickLook preview. And to avoid using the mouse entirely, you can use command-y at any time to bring up a QuickLook preview of all the attachments whether they are selected or not.
Textmate users, with the help of a plugin can use QuickLook from the project pane.
Users of PathFinder have full QuickLook support built in.
And, finally, QuickLook can even be used from the command line (see below).
Generating QuickLook previews from the command line