A seal is a device for making an impression in wax. The original purpose was to authenticate a document or the cover of a container or package holding valuables or other objects.
Melt the wax. Hold the stick in one hand and the match right at the end of the stick in the other and light. Give it a moment to burn down. Place the melting stick directly over the spot you want to make the wax seal and let the wax drip down. Press your seal into the melted wax, hold down until the wax sets. Once set, the seal will be easy to remove, while the wax with an impression will stick to the envelope or page.