Simon,
The way to tension an elastic is to thread it in the normal way, attach the the elastic to the bung. Then with a piece of elastic hanging from the tip stretch it so it bottoms out then let the elastic retract back into the pole. Now there should be a bit more elastic hanging out than before so without any tension hold the elastic at the point where it comes out of the tip and pull approx 3" out, this is the point where you will attached you connector or make the crows foot.
An even better way is to have a bung that has a tensioner built in, this way you can wind a bit more elastic onto the bung sorting out any 'droop'. It is also very useful for when the elastic gets a bit warn you can simply remove a few inches and tie on a new connector and release a few turns on the bung.
Job done, hope this helps