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.

Notice

When converting a DVD, be sure to use the option -udf instead of "joliet" to generate an UDF filesystem. This is particularly important with DVD video master images!