Flex Track is put together in the same way as other types of lego track. You can also use the 'F' shortcut key to add a Flex Track. The main difference is how the track can be "flexed" into shape after several individual pieces have been connected.
To flex a section of tracks (i.e. a Flex Chain) one must select the individual parts in section one wants to flex, then double click the track you want to grab and hold down the left mouse button on the second click. If this is done successfully, the mouse icon will change from a brick to an arrow with Flex Track symbol. While continuing to hold down the mouse button, the track can now be flexed within the parameters of the real Flex Track by moving the mouse in the desired direction of the flex. To stop flexing the track, simply release the left mouse button.
A Flex Chain can contains straight or curve tracks, it doesn't need to contains only Flex Tracks. Actually the Flex Chain will be built solely from the selected parts, so one can control how many parts should be included in the Flex Chain for a flexible move. Moreover, the Flex Chain will be built starting from the part you double click and following the connection links as long as the parts are selected and have only two connections. When a part with more than two connections is reached (track join, cross-over, level crossing, etc...) the Flex Chain is stopped.
The Flex Chain move use a convergent iterative algorithm. If you want to connect a Flex Chain to an existing track, you sometimes need to wait few tenth of second to let the algorithm find the optimal solution.