Pour your weak acid solution (lemon juice or white vinegar) into your glass container and add a small amount of salt. Now add your oxidised copper to the solution. Remember to use dull copper, as this is an indication that it's oxidised. The more oxidised copper you add the more chance you have of better results. Leave the solution to stand for at least 10 minutes.
While you wait it's a good idea to use your time to clean your iron nail. Clean off the top layer, using your wire wool. This will reveal the shiny surface and remove any impurities. Remember to give it a quick wipe when you've finished cleaning to remove any residual dirt. Once ten minutes has passed add a drop or two of fresh acid solution (vinegar or lemon juice which ever your using). Now add the iron nail and leave it at least half an hour or longer. I find swilling the solution around helps speed up the process.