I’m not sure how helpful this may be, but for those who make their own “serial Arduinos”, it could be.
When uploading a sketch, I was never sure when I should press the RESET button. I found that if an LED is put between the receive line and +5, it will flash once at exactly the right time to press RESET.
Rx --|<–///-- Vcc
I used a high value of resister so it would not interfere with the transmission.