For such sentence constructions, either the past tense “was” or the present tense “is” can be used depending on context and what the speaker or writer has in mind. For describing a past action that one actually did some time in the past, “was” obviously should be used, as in “What I did was to call the police.” On the other hand, for describing demonstratively (especially when making or pointing to a diagram or illustration in full view of the listener) a method or procedure that one performed in the past, the present tense “is” can be used, as in “What I did is this: draw a circle first, mark its center with a dot, then draw a straight line from that dot to the circumference.” In this latter case, the diagram or illustration usually becomes the real-time peg for the use of the present tense.
