Firmware V3 only works with the Web Client V3. It is not backwards compatible.
Step 2 - Connect the PCB to your Computer
Use the M0 connector to connect the PCB to your computer using a programmer.
Step 3 - Flash Firmware
After downloading the firmware, open ./SmartLight-Firmware/SmartLight-Firmware.ino with the Arduino IDE.
Make sure that your ESP8266-01 is connected to your PC and you choose the right Flash Size with at least 128KB SPIFS for your ESP8266-01.
My flash config for an ESP8266-01 with 1MB of Flash Storage.
Compile and Upload your Code.
Hold down both push buttons during the compile step in the Arduino IDE
release the RESET Button as soon as the IDE say "Upload"
release the FLASH Button as soon as the upload has started and you see a progress in percent in the IDE
Sometimes it is required to power off/disconnect the ESP/Programmer just before you upload the firmware.
Step 3 - Setup
When you Power On the ESP8266-01 it tries (for 5 minutes) to connect to the saved WiFi Access Point. While doing so he is hosting his own Access Point called "SmartLight - Setup".
Connect to it with your PC or Smartphone and open the URL http://192.168.4.1. The Password is "LightItUp" You must choose your WiFi there and can set up some other things like the type of LED-Strips you connected and what the hostname of the device in your WiFi should be.
Don't worry. If you ever want to change the config, restart the ESP and hold down the FLASH Button for >0.5s in the first 5s after restart. This will force the setup mode and opens the SmartLight Setup Access Point.
After Saving your settings, the ESP8266-01 should restart and the connected light should light up blue. When the ESP8266-01 established a connection to the given WiFi he will turn off all the lights and waits for your commands.