Turn on your Geiger counter.
Use the Geiger counter to take five readings without your element source present. Set the shielding and source to none if your Geiger counter has that functionality. Take the average of your five readings. For example, you may get readings of 50, 35, 70, 62 and 55 counts/min. The average is (50+35+70+62+55)/5 = 54.4 counts/min. This is the average background radiation.
Position your element source next to the Geiger counter. If your Geiger counter supports it, set the shielding to none, set the source position to how far away your element source is and set the source to the element you are measuring.
Set a timer for five minutes and take a reading from the Geiger counter every 30 seconds. If your element decays extremely slowly, you may need to take readings over a longer time period to measure a change.
Subtract the previously recorded average background radiation measurement from each reading.
Plot your adjusted readings on a graph with time (mins) on the x-axis and your readings (counts/min) on the y-axis. For more accurate calculations, plot the readings in a mathematical graphing program.
Draw a best fit curve through your data points. If using a mathematical program, have it draw the curve for you.
Estimate the point on the curve where the counts/min is half of the beginning value. If using a mathematical program, have it calculate the data point for you. This value is an approximation of the half life of the element. For example, your initial counts/min could be 100 and the counts/min could be 50 at three minutes, so the half-life is three minutes.