Knowing what ICM is, is one thing; its practical application is another. Unless you are some sort of maths genius you are going to struggle to do the sums in the middle of a hand.
What you can do is to look at hands (real or imaginary) with an ICM calculator to hand, and work out if you made to correct decision. For example, you have 12BB in SB in a STT, 3 people left, you have Axs and big stack button shoves for the umpteenth time. Using your best guess at bigstack's range (ATC?) you can work out whether the 60% times you double up are worth more in $ than folding. Do this exercise as many times as you can stand and you'll soon get a feel for how ICM applies in typical situations.