How did Hermann Ebbinghaus conduct memory experiments on himself?
Ebbinghouse indeed only tested on himself, limiting the generalization of his findings. 

Your question is a really good one; given the time of his writing, the now so familiar structure of a standard journal paper (Abstract, Introduction, M&amp;M, Results, Discussion) is not present in his monograph. However, tucked away in the text on p. 51 of 100 in this pdf version of his book he describes the following: 


  The investigations in question fell in the year 1879-80 and comprised 163 double tests.
  Each double test consisted in learning eight series of 13 syllables each [...] and then in relearning them after a definite time. The learning was continued until
  two errorless recitations of the series in question were possible. The relearning was
  carried to the same point; it occurred at one of the following times, -- namely, after about
  one third of an hour, after 1 hour, after 9 hours, one day, two days, six days, or 31 days.


As far as I understand, hence, he learned a sequence of syllables and then he basically tried to re-learn those sequences after a series of fixed times and see how his scores were the second time.