First install android-tools:

# pacman -Syu android-tools

Connect your Android device with a usb cable to your computer and run lsusb:

$ lsusb
Bus 001 Device 020: ID 0bb4:0f91 HTC (High Tech Computer Corp.)

Create and edit /etc/udev/rules.d/99-android.rules using the idVendor:idProduct of the Android device from the previous output and set OWNER to your username:

SUBSYSTEM=="usb", ATTR{idVendor}=="0bb4", ATTR{idProduct}=="0f91", MODE="0640", OWNER="leedev"

Reload udev to recognize your new rule:

# udevadm control --reload

adb, fastboot, and mkbootimg will now recognize your device.