The fundamentals is that we are looking for a certain radioactive element. We know that it has a certain half-life. All we need to do is find the actual content of the isotope in that sample. We can then find out the difference between the actual amounts of isotope in a live sample with the present content to find out the actual decrease.
Using simple math you can find out the years it took to reach the content it is present today, hence, the fundamentals of radioactive dating is the half-life of the isotope and the content of that isotope in the sample. You can then find the actual age with a simple calculation.