How to enable/disable location services on 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.
How to enable/disable location services on 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.
How to enable/disable the location feature on mobile.twitter.com:
(For browsers supporting HTML5 and geolocation.)
Log in to mobile.twitter.com from your mobile 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 the location feature?
Once location is enabled, post your Tweet as you normally would. Your location will appear with your Tweet, as highlighted below.