You draw a Right Triangle. Designate one acure angle as the angle of interest, call is S.

When contemplating sin(arccos(x)), notice that arccos(x) is an ANGLE. Let's call it S

If the cos(S) = x, then you can lable the leg adjacent to S, just "x" and the hypotenuse "1".

The the Pythagorean identity sugests the opposite side is sqrt(1^2 - x^2)

Simply write down sin(S).