If you don▓t have a diamond eye threader, a length of strong line (20lb plus) will do a similar job. The reason for doubling up the elastic for the few inches above the bung is that if you hook a big fish which charges off and bottoms out the No3 elastic, you have a short length of doubled-up No3 - effectively a No6 - to act as a buffer and hopefully stop the fish before it breaks your line!
Gareth Purnell, Improve Your Coarse Fishing

1.
With the telescopic top two sections together, push the threader or line in from the top. The superglue used to attach the bush should be dry and any surplus have been sanded clean before you attempt this.

2.
Push the top section down into the lower section and push through until the threader comes out the bottom.

3.
Lock the elastic into the ▒diamond eye▓ of the threader.

4.
Now pull the threader through your pole top sections.

5.
Pull the sections of the pole out until they lock in place and then pull the elastic through until you draw the bung into the pole.

6.
This bung comes with a special attachment, so no tag is needed.

7.
Just push the bung in gently until it won▓t go further, and twist off.