Since you won't be streaming video content all the time, it is crucial that you learn how to play FLV files offline. The first step is determining the media player compliant with this file format. A simple Google search should haul up tens of such media players, and the rest will be upon you. Popular players include:
AnvSoft Web FLV Player
This list is not exhaustive, so you may want to try other options. To play FLV content on your mobile device, download and install PlayerXtreme Media Player for Android and iOS. This app is available on Google Play Store and iTunes for iOS.
With the appropriate FLV player in place, you can now proceed to open and play FLV files. You can go about it in different ways. You could set your media player as the default player in which case you will be double-clicking on the FLV file and it would open in the default player.
But if it is not your default media player, you can drag and drop the file into the open window of the player. If everything is right, the file should play right away. Alternatively, right-click on the FLV video and select "play with" from the drop-down menu. A list of the media players available on your device will show up, and it is from this list that you will select your preferred player.
In some cases, your device may not be able to open and play flash videos even after installing a compliant media player. Such situations will force you to try other viable options, such as converting the files to a supported format.