How to practice and improve your Tango?
You can see here how we walk alternately in parallel and crossed system throughout the dance floor.
Here you will find a clear explanation of the meaning and differentiation of parallel and crossed system.
To choose a song and place elements in order accordingly?
While this is not a bad option, we suggest instead to create brief sequences of 2 or 3 elements that you can do in a small space, like the ones we usually get to do at milongas, so when you dance at milongas, you can use them, adapt and reshape them as you move on the line of dance.
This also applies for sequences of follower's embellishments.
Follower's can devise possible embellishments for each of the elements usually lead (backward and forward walk, ochos, molinetes, boleos, etc.)
Here is one element to start creating your sequences:
One time you have your sequences ready, study segments of them that could be reorganize as new sequences or as connectors between single elements.
Here you can find an example of the results of such approach: