As a minimum you should pre-tin the BT- pad with solder, it may take 3-4 seconds as there are no thermal reliefs on that test pad. It’s recommended to pre-tin the other pads as well to make the install quicker.
You should now plug in the bottom screen and power up the DS Lite to confirm the installation was successful. If you power up your GB/GBA/DS transmitter cart and press buttons, you will see the LEDs on the flex board light up.
It’s up to you if you wish to leave the LEDs as they are or cut the flex board to remove them as indicated by the white line.