windows clients in freeradius openssl

I have been trying to create certificates to secure my wireless infrastructure and cannot seem to get passed creating windows client certificates. The instructions that I am following on FreeRADIUS and Linux for Your WLAN say to create a file called xpextensions. What kind of file? Just a text file? I created a simple text file with the required data in it, ran the commands listed next and only run into errors. All I am trying to find out is what type of file this should be, and how to create it. Once I figure that out I think I can pretty much get it from there. Thanks.