It finally worked!!! I didn't knew about the DTR line. Connecting it to RESET directly didn't work, but then I found this: Programming Arduino Using USB to TTL without DTR pin - YouTube
In conclusion: To get the Arduino into a Flash Mode, I just had to hold the RESET button while compiling, and when it started the Upload, release the RESET button and it starts the transfer.
Thanks for pointing me to the right direction!