1. Repo sync all code with default.xml
   repo init -u git://github.com/OnePlusOSS/android.git -b oneplus/QC8998_O_8.1

   repo sync

2. Root your device

3. Pull the necessary libraries from your device by executing the script pull_library.sh

4. After pull_library.sh be executed, a directory named "vendor" should be generated. Copy this directory to root of project.

[Set up environment]
Initialize the environment with the envsetup.sh script. git a $ . build/envsetup.sh

[Choose a Target]
Choose which target to build with lunch.
$ lunch
enter 40 for msm8998-userdebug

[Build the code]
Build everything with make. GNU make can handle parallel tasks with a -jN argument.
$ make -j4

[Flash a Device]
Let device enter fastboot mode, run
$ adb reboot bootloader
Once the device is in fastboot mode, run
$ fastboot flash boot boot.img
$ fastboot flash system system.imgi $ fastboot flash recovery recovery.img

If you want flash new /data partition, run $ fastboot flash userdata userdata.img