Oakum is made from strands of hemp, jute or other materials used for making rope that is then soaked in tar, oil or grease to make it completely waterproof. Traditionally it was made from old ship's ropes and cables which were then laboriously "picked" apart by hand (often the work of prisoners or slaves) into their individual strands, then loosely tied back up and soaked in tar. Today, however, most oakum sold for use in wooden boats is specially made from new materials. "Caulking" is the term given to the process of sealing joints -- traditionally on a wooden ship to stop leaks!
Raise and clean the seam to make sure there is nothing in it that will compromise the seal and to ensure that as much sealing material has been stuffed in as possible. Open up the seam gently using a caulking iron, screwdriver or crow bar. Scrape out any foreign materials and clean the rest with soap and water. Allow to dry.
Apply caulking cotton. Once the seam is dry, jimmy it and wedge it open once again and apply the layer of caulking cotton. Shove the caulking cotton as deep as possible using the caulking irons or tools so that it fills in the first third of the seal. Pack it in tightly.
Apply the oakum. The oakum should be packed in as tightly as possible above the cotton along the whole length of the seam.
Seal with tar, pitch or similar compound. Make sure you use a compound that is suitable for the job at hand. If you are sealing a joint below the waterline, use a sealant expressly designed for the purpose. If you plan to paint over the seam, use a sealant conducive to this. The neatest way to apply this layer of the seal is using an applicator gun. Should you wish to be more traditional, you can use a funnel, or even a trowel or spade. Make sure that the tar is packed in tightly and creates a seal on both sides of the seam. Tidy up the seal on either side of the seam so that it makes a nice, neat line. Leave to dry.