How do you actually use Vats and Crits?

Question for you PS4 vats players from someone who doesn't use vats but is looking to do so. How do you bank and use crits on PS4? Be specific I have been running melee and vats has pretty much been useless and broken for so long that I don't use it at all and see people post how they bank and use crits but I haven't the slightest how you do that. Guessing there is a specific key that uses the crit or stores it and then lets you use it? But have been thinking of doing a vats rifle build and would like to learn the process from those that are in the know. Thanks in advance.