As other have said, not easy and that is an understatement!
First, the electrical interfaces are different and not compatible:
COMM (Serial) is +/-12V with Tx, Rx and 0V lines
USB is 0V/5V on differential pair (D+ and D-) with 0V
Then you have the protocol that each talk?
Serial is entirely propritery by the application. Only the protocol bit level is configurable (baud rate, no of bits per byte, parity etc)
USB has multiple layers. First there is the basic USB control layer, where host and device must first hand shake and then agree what the device type is.
There is a device type class for printers. Since printers vary so much, much of the next layer up (actual transfer of image data) is almost propritery to each manufacturer and device.
Most manufacturers now use postscript or PCL (HP rival to postscript)
However, within these common protocols, there are manufacturer specific sections which the printer will use for parameters which the standard protocol does not cover.
I am not saying that it is not possible - just very very very very difficult.
Drop using COMM port, many microcontrollers now have USB peripheral devices to interface to usb.