Hi, welcome to my instructable.
So on this tutorial will show/tell you how easily flash an Arduino without USB to Serial (TTL) adapter with only another Arduino that has USB Port onboard.

What did you need?

Arduino that has USB Port (Mega, Uno, Nano, etc)
Arduino that requires a serial adapter (I'll use Pro Mini)
USB Cable
Some jumper wires

Difficulty?
1 out of 5

Step 1: Prepare and Connect Everything!

Before connecting all cables together, You need to remove the IC on the Arduino that has USB onboard, then you are ready to flash the Arduino that require Serial adapter.

On this tutorial, I use Nano and Pro Mini. You can use anything you want.
For 5V variants, connect VCC to 5v pin. IF you had 3.3v, connect it to 3.3v pin from VCC.

How to connect the cable?

Make sure you have been removed the Microcontroller IC on main Arduino
Connect these pin from main Arduino to the Arduino that you want to flash :

Pro Mini > Nano

TX > TX
RX > RX
GND > GND
VCC > 5V (My board is 5v model)
RST > RST (or you can just simply press the reset button when finished compiling and start uploading)

Step 2: Change the Configurations First!

Before you start flashing the Arduino, make sure you've change the parameters for Arduino models and variants.
If you skip this step, you cannot flash the Arduino.

What configurations should You change?

Board Model
Microcontroller type and operating coltage (e.g ATMega328/ATMega168, 3.3v/5v)
Communication port number

After you change the configurations, you can upload the code!

Thanks for read my useless instructables!
Even it's useless, but it works!
Check out my YouTube channel for more interesting DIY Projects!
Have a nice day!