Try looking for a tutorial in youtube?
The process is basically something like this...
Insert one end thru so it protrudes about 2 mm. Heat that protruding tip with a flame and as soon as it melts and forms a molten bulb, give it a slight tug backwards and it will sit in snugly on that end. The melted bulb will form the first anchor.
Now, snip off the other end, again leaving about 1-2 mm. Heat up this other end. What should happen is that the melting end starts to create tension and tighten on itself, making a nice straight fibre installation. Do not get any heat on to the centre of the fibre or you will just deform it. Hope this description helps with ypur reinstallation.
I can't be precise about how much to excess length to leave but if you estimate it right, it will work out. About 1 to 2 mm. If its too much, the fibre wont straighten and the bulb will be too big. If it's too little, it might not work... a little trial and error mighy be required?