Chemists often determine the relative proportions of isotopes in a given sample by using a special tool called a mass spectrometer. However, at student-level chemistry, this information is often provided for you on school tests, etc., in the form of established values from scientific literature.  For our purposes, let's say we're working with the isotopes carbon-12 and carbon-13. Within a given element, different isotopes appear in different proportions. These proportions are almost always expressed as percentages. Some isotopes will be very common, while others will be very rare - at times, so rare that they can barely be detected. This information can be determined through mass spectrometry or from a reference book. Let's say that the abundance of carbon-12 is 99% and the abundance of carbon-13 is 1%. Other carbon isotopes do exist, but they exist in quantities so small that, for this example problem, they can be ignored. Multiply the atomic mass of each isotope by its percent abundance (written as a decimal). To convert a percentage to a decimal, simply divide it by 100. The converted percentages should always add up to 1.  Our sample contains carbon-12 and carbon-13. If carbon-12 makes up 99% of the sample and carbon-13 makes up 1% of the sample, multiply 12 (the atomic mass of carbon-12) by 0.99 and 13 (the atomic mass of carbon-13) by 0.01. A reference book will give percent proportions based on all the known amounts of an element's isotopes. Most chemistry textbooks include this information in a table at the end of the book. A mass spectrometer can also yield the proportions for the sample being tested. Sum the products of the multiplications you performed in the previous step. The result of this addition is the relative atomic mass of your element - the average value of the atomic masses of your element's isotopes. When discussing an element in general, and not specific isotopes of that element, this value is used. In our example, 12 x 0.99 = 11.88 for carbon-12, while 13 x 0.01 = 0.13 for carbon-13. The relative atomic mass of our example is 11.88 + 0.13 = 12.01.

Summary:
Determine which isotopes are in the sample. Determine the relative abundance of each isotope in the sample. Multiply the atomic mass of each isotope by its proportion in the sample. Add the results.