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Nova Settings -> Gestures & Inputs -> Double Tap
Then you have a few choices here. In the Nova Launcher beta, they have a sleep/lock action built into the launcher.
For stable Nova Launcher, you can either use the GravityBox Action 'Go to Sleep', or you can get an application from the Play Store called Screen Off and just set the gesture to that specific app
I use Screen Off, you can add it to your favorites and tap the icon to instantly lock your screen.
You can also assign the App to act as your assistant; this way you can lock your screen by holding the home button.
>I'd like to be able to turn the screen off from any application.
My app does this with a Quick Settings Tile. I hope you like it!
https://play.google.com/store/apps/details?id=nl.matthijsvh.screenoff
You can use an app like Screen Off to turn the screen off. You can put it on home screen. And wake the phone back up with home button.
Ok, now I have a new scheme (tried the create another user account and decided that was too much).
So now I have smart lock set up and operational. I learned that when you have the lock screen in front of you, you swipe up to wake up the phone. If you want to use Android Pay, don't swipe up. Long press the unlock button at the bottom center and that will give you a locked screen to unlock.
I've actually installed Screen Off as a back up way to get to a scree that requires my pin that Android Pay will like.
These are the two I tried right before Nova integrated the feature
I don't know if these are any less sketchy. . .but they are the ones I remember using
A work around would be to install screen off or similar. And then change the quick tap gesture to open it.