I’m interested in making a USB to serial converter?

would it be easy or hard to design my own USB to serial converter? Is it a wrather difficult task?
a cable..that is

I take it you mean a USB to RS232 converter. Not hard. There are chips available.
http://www.pccompci.com/usb-converter/usb-converter.html
USB to Serial Port for AEM EMS / ECU / Stand [...]