Put all the items you want in the ISO into a folder
Launch disk utility
From the Menu Bar go to Images>New from folder
Select the folder with the items
Enter a Save as name
And Image Format must be DVD/CD master
The resulting file will be filename.cdr, which you can rename to .iso
(They're the same thing, that's what the DVD/CD Master are for. If you chose read-only renaming it ISO won't work on a PC because it'll make a HFS+ volume instead of a CD volume, I believe.)