1) Download Tor Browser - https://www.torproject.org/projects/torbrowser.html.en
2) Download the User Agent Switcher for Firefox - https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/
3) Update/Import the updated user agent list - https://github.com/miketaylr/useragent-switcher-xml
4) Go to Applications (if in Mac) right click on Tor Browser and do "Show Package Contents" - Navigate to Data > Tor > Open torrc-defaults in a text editor
5) Once the file is open in text editor add the line "ExitNodes {AU}" (without quotes). Change {AU} to whatever country code you need to test
6) Launch Tor Browser and change your User Agent to whatever type you want to test - for example if you're testing an iOS offer in Indonesia your torrc file would have {ID} and your User Agent would be Safari iOS7