Ars Technica describes
the ritual that will be required before a future Android device will
deign to install apps from somewhere other than the Play Store.  It is not
for the impatient.

Here are the steps:

Enable developer options by tapping the software build number in About
Phone seven times
In Settings > System, open Developer Options and scroll down to
"Allow Unverified Packages." 
Flip the toggle and tap to confirm you are not being coerced
Enter device unlock code
Restart your device
Wait 24 hours
Return to the unverified packages menu at the end of the security delay
Scroll past additional warnings and select either "Allow temporarily"
(seven days) or "Allow indefinitely." 
Check the box confirming you understand the risks.
You can now install unverified packages on the device by tapping the
"Install anyway" option in the package manager.

https://lwn.net/Articles/1063735/
--- SBBSecho 3.37-Linux
 * Origin: Palantir * palantirbbs.ddns.net * Pensacola, FL * (618:250/24)