Create a folder with the contents you want on your ISO.
Open Disk Utility and use the New Image from Folder menu item to create an image. Ensure it is uncompressed and use the CD/DVD master option.
In my experience this creates HFS+ masters which are no good in Windows.
Open the Terminal
Assuming your new image is called ~/Desktop/Master.cdr (the file is on your desktop and called Master.cdr) type:
cd ~/Desktop hdiutil makehybrid -iso -joliet -o Master.iso Master.cdr
This will create an ISO/Joliet .iso file.