As stated above you need a Navigation Computer then follow the instructions on the front of the device:
SET Corected OAT against Arrow, or Indicated OAT against Mach No (In Mach Window) READ TAS on Miles scale against Mach No on Minutes Scale.
Its that easy no maths involved.
Very hypothetical when you're bimbling around in a Cessna
Lets say you opt to fly at 10,000 ft then the TAS will exceed the IAS by 15% so it is in your interest to know how to work this out.