Put a little heat on it with a lighter or match....careful...not too much, to get the old one off, take it to Academy or anybody that might have one so you are sure to get the right size. Get some hot melt glue and put the new one on there. Academy usually has a repair kit with 3 tips with one that will usually cover the size you need and has the hot glue in the package. If you want a higher quality guide go to mudhole.com and order one if you can figure out the size.
Again...be careful with the heat. Too much will turn that graphite to dust.
"Be as straight as you can be and as crooked as you have to be, for what's right"