IPA: /jan ˈstɛ̃.pʲjɛɲ/. Plain English will be tricky, but we can get there with a touch of French. Let's split “Stępień” into two constituent syllables: “stę” and “pień”. The stress is on the first one. “Stę” is like “stack” but you replace “ack” with the nasal vowel in “vin”, “wine” in French. To pronounce “pień” take the “p” from “pearl” and attach “yen”, the Japanese currency. Lastly, replace the final “n” with the “ny” sound from “canyon”. How does it sound? I'd love to have your recording.