You write equations in the usual algebraic form. You don't write RPN on paper (or screen), that's not a thing.

When you're going to compute a value on the calculator, that's where you use RPN starting from the innermost computations working outwards, because you have a stack to keep all the intermediate results handy.

Again for like 2+2 it doesn't make any difference, but if you have a long formula with lots of values and different operations, it is much easier to do correctly and fast with RPN.