Once you start building something with microcontrollers, one thing you need to take in to account is programming adapter. This is a device which allows to upload compiled code in to chip. I don’t know if this is still a fun to build your own DIY programming adapter which is not guaranteed to support all chips nor it will be safe and reliable. AVR microcontroller niche is one of most interesting when talking about programmers. If you take a look at AVRDUDE configuration file you will find that there is about 50 of them. Many of them are DIY while other 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.