On this page of the Disco Elysium game guide, you will find a detailed walkthrough of the Find smokes and smoke them task.
In Disco Elysium, the task Find smokes and smoke them isn't complicated - simply acquire a pack of cigarettes by any means of your choice, then let Harry relax for a while.
A pack of smokes can be bought at the Frittte kiosk, for example. Place the smokes in Harry's hand (left or right), then pull one out of the pack and have him smoke it.
After lighting the cigarette, Harrier will immediately lose 1 health point, so make sure beforehand that he won't die as a result.