Cigarette Butt
Tobacco Ashes
This is a combination of the words 吸う meaning "to smoke" and 殻 meaning "shell" or "husk." The husk left over after you're done smoking is the cigarette butt. Or tobacco ashes if you just have a pile of them.
すいがら
This is a combination of the words 吸う and 殻. Change the う to い and don't forget the rendaku, and you should be able to remember this reading too!
吸い殻が二本たまったら、すぐに灰皿を交換してください。
If two cigarette butts pile up, please empty the ashtray immediately.