I do it the same way mhgy does: "wherein adding detergent comprises: ..."
Incidentally, I was trained to avoid "step" in method claims (and to avoid discussing the "steps" of a method in responses to OAs). This was ~10 years ago, and I don't recall what the rationale was, but it's a habit I haven't broken, and I haven't had any bad outcomes from not using "steps."
” invokes 112(f) [pre-AIA 112 p.6]. Some practitioners feel that “steps of
” is too readily confounded with “steps for” and therefore avoid it. I’ve never had an Examiner invoke 112(f)/112 p.6 for “steps of”; and I believe that a CAFC case ruled that “steps of” does not invoke 112(f)/112 p.6 [I checked into that many moons ago, so I’m not certain at this moment]. Nevertheless, some in-house counsel prohibit “steps of”, while other in-house counsel (even at the same corporate client) insist on “steps of” [probably depends on what they were originally taught at the beginning of their careers]. In these instances, I follow the directives of in-house counsel. Left to myself, I prefer “steps of” simply because I think the language reads smoother, particularly in the OP’s instance of referring back to a previous step.