What this step is. With the WPSD image written, you'll put the card into the hotspot and turn it on for the first time.
1. Insert the SD card. With the hotspot unplugged, find the microSD slot and slide the card in (gold contacts toward the board) until it's seated. On most units it clicks; on some it just slides snugly in.
2. Connect power. Plug the power supply into the hotspot, then into the wall. There's no on/off switch — it powers on as soon as it has power.
3. Let it finish its first boot. The very first boot takes a few minutes — longer on a Pi Zero. WPSD is expanding the card and setting itself up, and you'll see the board's lights blinking. Don't unplug it while this is happening; just wait.
What “ready” looks like. After a couple of minutes it settles down and the dashboard is up and running, ready to configure. That's the next step.