How Do We Index Address

So I ran up against a wall trying to edit esper level-up bonuses because index addressing seems so simple, but does not compute.  This thread is for the posterity of figuring it out.  From my readings, index addressing is simple.  Instead of going to a static location, you go to a pointed location that is instead increased (or decreased?) by one of the other registers (X, Y, etc.).  So (LDA, X) JSR ($CA24,X) means you jump to the address but if 2 is the current value in the X register, then you go to that address +2.

This is how index addressing works, isn't it?  When I try to modify things in FF6 based on this it doesn't work out.  Also, I often can't follow the C2 notes that are based on index addressing, as they look to be using it differently than I imagine.

Thanks All,
RS

Edit: I did mean JSR for that example.  (LDA, X) is loading to the accumulator like you all said