The best way to apply plumbers' putty is to roll a rope of the putty about the width of your finger and then press it into place on the fixture you want to seal.
It should be pressed firmly, but not so hard that you damage the surface or create too much pressure. When finished, any excess putty can be trimmed with a putty knife or scraping tool.
Once applied and cured, plumbers' putty should be inspected periodically for signs of damage or deterioration.