JStark1809 notes in addtion to Incarbonites instructions:

If you want to use a JG/Golden Eagle M4 fire control group in your FGC-9 build, you will need to make sure that you are using the following components with it.

Following parts from your airsoft fire control group:

- Trigger
- Hammer 
- Disconnector 

Components for/from actual AR-15 rifles:

- Trigger spring 
- Hammer spring
- Disconnector spring  

- Fire selector  

- Fire selector detent
- Fire selector detent spring

You can buy the AR-15 components on AliExpress.

You can find the parts in parts kits that often are listed as: "AR15 Lower Parts Kit 223 / 5.56" "Kit All Lower Pins, Springs and Detents .223/5.56 AR15 Parts"

Example listing: https://www.aliexpress.com/item/33008262340.html