Choose a sensible name for the source code file. Let's assume here that you call it “mydriver.c”

Put your new driver into comedi/drivers/mydriver.c.

Edit comedi/drivers/Makefile.am and add mydriver.ko to the module_PROGRAMS list. Also add a line

mydriver_ko_SOURCES = mydriver.c

in the alphabetically appropriate place.

Edit comedi/drivers/Kbuild and a line according to the type of driver:

obj-$(COMEDI_CONFIG_PCI_MODULES) += mydriver.o  # for a PCI driver

obj-$(COMEDI_CONFIG_PCMCIA_MODULES) += mydriver.o  # for a PCMCIA driver

obj-$(COMEDI_CONFIG_USB_MODULES) += mydriver.o  # for a USB driver

obj-m += mydriver.o  # for other driver types

Run ./autogen.sh in the top-level comedi directory. You will need to have (a recent version of) autoconf and automake installed to successfully run autogen.sh. Afterwards, your driver will be built along with the rest of the drivers when you run make.

If you want to have your driver included in the Comedi distribution (you definitely want to :-) ) send it to the Comedi mailing list for review and integration. See the top-level README for details of the Comedi mailing list.) Note your work must be licensed under terms compatible with the GNU GPL to be distributed as a part of Comedi.