The last step to use a ATMega328P / Yabbas from the Arduino IDE is to plug it with a FTDI cable (TTL RS232 over USB). Personnally, I’m using the TTL-232R-3V3 to do it. The first step was to create an adaptor to transform this cable output to the YABBAS pinout.
Recently I made a Yabbas arduino system ; this is a low cost arduino (7€) you can DIY. I’ve made it with a group of people from my city (look here) and my daughter. During this session we have finished the prototype but not had the time to flash the AtMega328P.
Now this is done using my Arduino Leonardo as an ISP programmer. I’ll detail the way I’ve done it here