Rashi did not give a formal introduction to his methodology. However, this Likut takes key sources from Rashi’s Perush to explain Rashi’s particular understanding of Pshat.
The Pshat of a word in Rashi is not the “simple” or “plain” meaning. It is the meaning as can be understood by the use of the word somewhere else in Tanach. If the word only appears once in the Torah, then the Pshat cannot be determined.
Please click on this PDF to see the Likut. It should be printed out and studied.
Rashi – Peshuto Shel Mikra 6 2nd-1
Addendum: These sources follow a shiur given by Rabbi Jay Miller, z’l, in which he explains his unique analysis of Rashi.