A primer is a molecule that binds to a particular sequence of DNA. Once bound, the binder can be detected to determine whether or not that sequence was present in the sample. This allows for testing of specific sequences that correspond to a certain genotype. Once you have selected a primer that binds to the sequence in question, you will need to extract DNA from the cell. Follow the appropriate extraction protocol for your lab. Once you have collected a sample, you can test it. Add the primer to the DNA sample. If the sequence corresponding to that primer is present, it will bind to the molecule. Once this is complete, you can move on to analysis. For simple cases, the results simply come back as positive or negative based on whether or not the primer was bound to the DNA strands. Some more complex methods may require post-PCR procedures. These procedures can be lengthy and expensive, and are avoided when possible.

Summary:
Select a primer. Collect a DNA sample. Add the primer to the sample. Analyze the results.