Choose a calibrator sample to serve as a baseline. If you are looking for a particular coded gene product, or specific RNA, then you should choose an organ or organelle that will have the least amount of that product. If you're looking for human c-myc mRNA, for example, use brain samples as calibrators.
Gather your data from your sample and your calibrator. This process will vary widely, given the extreme diversity of species that pass down genetic coding from one generation to the next. In the example of human sampling, you would obtain organ samples from a scientific supply house and apply the reagent specific to that organ. After the reagent reacts with your sample, you will be left with a concentration value for that sample, and another value for the calibration sample.
Generate a spreadsheet using Microsoft Excel that has the following columns: Sample Type, nanograms of the target RNA, the logarithm of that value of nanograms, and the threshold cycle, also known as C(t). This last value indicates the point at which the sample gives enough of a fluorescence signal to be detected. The lower this C(t) value, the higher the concentration of the RNA that you are looking for.
Use the logarithm of the nanograms and the threshold cycle as your x- and y-values in an XY scatter plot, using the chart wizard in Excel.
Select one of the data points at random and click on it. Then, go to "Insert" and "Trendline" to run a line through that point. Go to "Type" and choose "Linear Regression."
Choose "Options" and then mark the boxes next to "Display Equation on Chart" and "Display R-Squared Value on Chart." This chart will give you a curve with the relative gene expression, showing you the relative concentration of the RNA in each sample.