Single post in Guide to CSS: Beginner to Expert

Hello! So I would like to display a count-down of how many gems I will need to reach Wishalloy in a specific type of badge, but I'm not sure how to go about it. Ex: x,xxx gems left. I see plenty of other users utilizing a similar display for their goals and whatnot, and I'm having trouble figuring out how to execute it myself. I figured it might be utilizing some style-coding. Could someone give me a run-down of the general scripting used? Thank you kindly. ^_^