To enable/disable the location feature on your mobile device:
For Twitter for iPhone and iPad:
Go to your iPhone or iPad's Settings app.
Tap Location Services
Tap the on/off switch for the Twitter app.
For Twitter for Android:
Go to your Android phone's Settings.
Select Location or Location & security from the menu.
Make sure that GPS location services and/or Use wireless networks is enabled.
The Twitter for Android app will then automatically have access to your location. To turn this off you will have to disable your phone's GPS services.
For mobile.twitter.com (for browsers supporting HTML5 and geolocation):
Log in to mobile.twitter.com from your phone.
Tap the Tweet button.
You will be see a pop-up that says https://mobile.twitter.com would like to use your current location (pictured below).
Select OK. (Selecting Don't Allow, means you do not want your location to be seen by your followers, and will prevent you from being able to use the feature.)
Once you've selected OK, you will see your location just below the box where you type your Tweet.
You can turn the location feature on and off by tapping the location button on the bottom of the Tweet box (also pictured below). The button will be highlighted when the feature is enabled and gray when disabled.
How do I use it?
Once location is enabled, post your Tweet as you normally would. A small location pin will appear just below your Tweet, as shown below.
Anyone viewing your Tweet will see a map of the place you were when you posted the Tweet.
If you don't see the map, don't panic! Some browsers take a while to load completely.