@Griffiths415 - My apologies if I’m telling you things you already know, but you don’t need to run “Hello, Home” to make this work. You need (1) to configure the switch that controls your outside light, and (2) a working geo-fence detector.
For (1), when you select the settings gear on the “Thing” that controls that light, and choose the SmartApps tile, it walks you through the Thing setup. The fourth menu is “Customize and control the [Thing that is your outside light]”. Near the bottom of that menu is an option saying “Turn on when people arrive”. When you select that option, you can configure it to turn off a few minutes later, and only to turn on between certain hours, or in certain modes.
If you want your presence to trigger more events than just turn on a single light, you’re probably better off having the presence detector trigger a Mode change, and then have the mode change trigger all the different events you want (e.g., change the thermostat, unlock a door, start music).
For (2), I’ve read that people using their mobiles for geo-fence detection have struggled. I have a SmartThings Presence Detector on my keychain and I’ve found I still need to set the geo-fence to be very large (in the manner that @antman2 describes above).