Recommended · for some experience
1-Day Project: Build Your Own Arduino Uno for $5
Why people recommend it
Julian Ilett builds a standalone ATmega328P Arduino for about five dollars, a common starting point for people making their own Arduino-compatible boards.
Watch for: upload problems several builders report when programming the finished board
In their words
There is a video on how to achieve, more or less, the same thing, 1-Day Project: Build Your Own Arduino Uno for $5.
Greenonline on arduino.stackexchange.com, Jul 2015. CC BY-SA.
I have made a standalone arduino board with a Atmega328p w/ Uno bootloader taking inspiration from this video [video] However, when I try to program mine all I get is "Problem uploading to board".
Gregulimy on arduino.stackexchange.com, Aug 2017. CC BY-SA.
I've made a home-made Arduino based on this video.
haxpanel on arduino.stackexchange.com, Mar 2016. CC BY-SA.