This is an easy one. But worth discussing because Inspiration is a completely new concept to DnD as of 5e and one of the few entirely metagaming rules out there.
Inspiration is a reward the DM gives to a player for doing something consistent with his personality, background, alignment, etc. The player than can elect to use his inspiration to gain advantage on any dice roll. It can only be used once until the DM awards you inspiration again.
So Inspiration is either on or off. And on the character sheet, that’s all it is: an on/off switch.
If the DM awards you inspiration, just click the box and it turns on. When you use it, turn it back off. That said, the RAW are a bit vague on when exactly you declare using inspiration. For this campaign, our house rule is if you fail a save, you can then say “I want to use my Inspiration and re-roll it.”
Phew!
Curse of Strahd 