Use a very little bit of oil, like a drop on your finger run it around the tube where you want to break it, then score it with a file edge where you want it, you just need a light scratch to create a weak spot in the glass, then in the middle of the tube, score a small hole in the lamp with the file to equalize the pressure of the near vacuum or it will cause it to rupture unpredictably, remove the oil with glass cleaner, then wrap it in a old news paper, put it in a large trash bag and whack with a hammer in the middle of the tube, you should then be able to then just tap your score line with the file edge to get a clean break and use sand paper the get a smooth edge. But if it's for a display I would suggest practicing on some junk EOL tubes to see how it goes, a Small triangular files I find works the best.

Now the word of warning some older tubes contained some very nasty chemicals like beryllium oxide in the phosphor (real nasty stuff which for the most part has been replaced with other not so nasty chemicals) and all contain mercury, so be careful with the disposal of the waste product and do it outdoors or use proper protection or even better both...