So my enemy has a +18 to Will saves, I'm using Demoralize via Performance and its +23. How much do I need to crit success vs them? I know it's +10 but is that after they roll a D20 then they add +18 to that and I roll a D20 + 23? Or?
Your target doen't roll its save. It uses its Save DC which is 10 + its save value; in your case 10 + 18 = 28.
So you need a result of 38 to crit. With your bonus of +23 that would happen if you roll a 15.
Basic rule: Only one side rolls, the other side provides a static DC.