The key should be be formatted the same as the rest of the transcription--double-spaced, and uncontracted. There will be no need to add extra lines where the texture symbols occur as they already take two lines. However, the alphabetic key would be double spaced. If it is only 1 page long, it will be a left-hand page facing the tactile graphic on the right. However, if the key becomes more than one page then you would need to follow the Exception rule 184.108.40.206 whereby the tactile graphic is placed on the left side, with the key following, all as right-hand pages.
Hope this is clear enough. See also the example on page 5-31 of Guidelines and Standards for Tactile Graphics.