Once you start building something with microcontrollers, one thing you need to take into account is a programming adapter. This is a device which allows uploading compiled code into the chip. I don’t know if this is still fun to build your DIY programming adapter which is not guaranteed to support all chips nor it will be safe and reliable. AVR microcontroller niche is one of the most interesting when talking about programmers. If you take a look at the AVRDUDE configuration file, you will find that there is about 50 of them. Many of them are DIY while others are official. When I started with microcontrollers I also was looking for cheap and easy to build programming adapter. So I have built several if few years.