how do i create an OS X 10.6.8 installer disk?

You can't. If you don't already have Snow Leopard you will have to buy the disc from the Online Apple Store.

If your Mac came with Snow Leopard and you have the disc but want to install it on another machine you will still have to buy the disc from the Store - the disc that came with your Mac will work only with that.

If you have Snow Leopard installed but have lost the install disk you can obtain a replacement at a moderate cost by ringing the Store at the number given at bottom left of this page and quoting the serial number of your Mac. This disc will work only on that Mac.

Just FYI - Unfortunately, buying replacement system restore disks doesn't seem to be an option any longer. Just called Apple and gave them the serial number of my 2011 iMac, which I bought on eBay and of course doesn't have the original disks.

The rep said I could not buy the restore disks from Apple, but I could use the Retail version, 10.6.3, and then combo-update to 10.6.8. I told her, "People on forums all over the interent are saying that a machine that was built and shipped after 10.6.3 was released won't work with the retail box version" (would lack specific drivers for that machine). She assured me it would.

I don't trust that answer though, so I just bought a 10.6.6 imac-specific restore set on eBay. If it works, I'll at least have the iLife software too.