As you can see, it's not brain surgery. You just thread everything together like below, hold the bottom nut while you tighten the top one, and the headset smooshes right in. Two tips: grease the headset cups and head tube (duh), and tighten slowly, checking the cups for straightness as you go. If a cup starts to tilt, tap the threaded rod toward the edge of the cup that needs to go in farther. I squeezed both cups in simultaneously, but it's best to install one and then the other. Follow my own advice? Nah.
Two minutes later, the headset is installed. If you're wondering, it's an inexpensive Necco unit from Porkchop BMX. Just $20, with a nice finish, aluminum compression ring (red anodized no less) sealed cartridge bearings, and a contact seal between the dust cover and top cup. Only downside: no silver option, but the gold looks pretty good with the NTF's dark metallic green paint. About that stem: it's just there to keep the fork from sliding out.