I select "Smart Home" in the Alexa menu, and the new Stringify flow shows up in the list as if it were a device. If I don't see it yet, I select "Add Device", Alexa says "Discovering new devices..." and scans for a new device under any of the installed skills. It is usually able to pick up the new Stringify flow after about 15 minutes.
In the event of a service connection failure Alexa can be quite slow to update. For example Honeywell's Home service that connects my smart thermostat goes down frequently (way too frequently!). Google Home is always the first to pick it up when it comes back online, then Stringify and Alexa follow up to 12 hours later.