This page attempts to document the steps necessary for creating a custom ROMDisk image, creating the resulting ROM image, and burning it to a ROM SIMM.
Creating the Disk Image
The disk image is a flat image like those used by minivmac or Basilisk II. The format is also used by the Serial Disk driver, Cloud 7 (HTTP Disk) driver, and the nubus_image driver of MESS. You can use any of those to create or update your disk image, but this will focus on using minivmac.
Here are premade blank images for 1.5MB and 7.5MB. If you want to create your own, here's how: To create the appropriately sized disk image on OS X, Linux, or a UNIX-like system you can use the following commands:
dd if=/dev/zero of=romdisk.img bs=1 count=7864320
This will create a 7.5MB file named romdisk.img. If you are using a 2MB ROM SIMM, you will want to create a 1.5MB image file, so use count=1572864 instead.
Next, obtain disk images for the OS you want to install. Apple provides some older systems for download here. You'll also need some way of decompressing these images. Using Stuffit on older versions of OS X, or Classic should work.
Finally, obtain minivmac for your platform and start it up. There is some good documentation on getting started with minivmac here. It should launch to a flashing '?'. From here, press control-o to open a disk image. Select your OS install image. It should then boot to the desktop. Attach the remaining images for your OS using control-o and selecting them all, so they're all attached. Next, attach your romdisk.img file. It should tell you this is not a Macintosh disk and offer to initialize it. Click 'Initialize', give it a name, and it should mount on the desktop.
From here, you should be able to install your OS onto the disk image like any other disk.
Remember, the image will be burned into ROM and unchangeable, so you'll want it exactly as you intend, with the right volume name, whether windows are open, window placement, icon placement, etc. Additionally, when you are done installing, it is a good idea to boot minivmac from your disk image, if you've installed the "System for any Macintosh" (as in, if the installed image has support for the Mac Plus that minivmac emulates). Booting from the image will ensure the desktop file is correct for the installed system.
Creating the ROM Image
Once the image is created and configured the way you want it, we need to concatenate it with the actual ROM image into a single file that will be burned into the ROM SIMM. Obtain the ROM image you want to use, such as those available for download on the ROM Disk page.
The disk image just immediately follows the ROM image, so this is a simple concatenation operation to merge the two into one image. On OS X and UNIX-like systems, issue the following command:
cat iisi+romdrv0.9.4+nomem+nosum-8mb.bin romdisk.img > rom.bin
The resulting rom.bin file should be the exact size of your ROM SIMM.
Under OS X, there really isn't any setup or installation to be done. The programmer presents its self as a Communications Device Class (CDC) USB device, aka modem or serial port. No separate driver installation is necessary. When connecting the programmer, System Preferences may launch telling you it found a new network interface (modem). Whatever you do at this step is fine (canceling or accepting) and won't affect operation of the programmer or its application.
Programming the ROM SIMM
Obtain the ROM SIMM programmer application here
To program the ROM SIMM, first ensure the drop down "SIMM capacity" has your SIMM selected. You only care about the size on the right side, either 2MB or 8MB.
Next, under the "Write file to SIMM" section of the window, click "Select file..." and select your rom.bin file. Then click "Write to SIMM".
Congratulations, you have a custom ROM SIMM