This is how I installed the Pixel Launcher on my OnePlus One running LineageOS 14.

First, download the Open GApps Stock variant on your phone.

Second, create the following directory and file on your phone:

 /sdcard/Open-GApps/gapps-config.txt

Put the following in it:

Include

CalculatorGoogle       # Install Google Calculator
ClockGoogle            # Install Google Clock
Maps                   # Install Google Maps
TagGoogle              # Install Google NFC Tags
Photos                 # Install Google Photos
GooglePlus             # Install Google+
Hangouts               # Install Hangouts
YouTube                # Install YouTube
Search                 # Install Search
Wallpapers             # Install Wallpapers
PixelLauncher          # Install Pixel Launcher (requires Search and Wallpapers)
PixelIcons             # Install Google Pixel Icons

This will basically tell GApps to install the Mini variant and the Pixel Launcher, since there isn't enough room to install everything included in the Stock variant.

Reboot your phone to recovery, and install the GApps Stock zip file as usual.

More info on configuring GApps is available here.

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