I honestly think you can make someone a baby leftist by just avoiding all the hot button issues that would sidetrack your discussion like China, the USSR, etc. Pointing out the obvious failures of capitalism, if the person you're talking with can relate to them at all, should leave them in a position where they can at least accept that capitalism is unsustainable. From there it would take a while to get them to be an actually productive leftist, and it probably won't be you who does it. But everyone I've got over on our side, it always started with a conversation about how the status quo is extremely unsustainable and produces terrible results for everyone around the world, not a screaming match on a controversial topic.