That's simple. From Burn.app's menu select 'Tools/Burn ISO Image', which will pop up the following dialog box:

Select the ISO image to burn.

Decide whether to change the burn parameters before burning.

Start burning.

You can select only one ISO image et a time. Burn.app does not care for the file name extension of the image. It will accept all file names.

You may as well use the offered service 'Burn Image To CD' from those applications that can supply the appropriate pasteboard data, e.g. GWorkspace.app (see also 'Services.../Burn Image To CD').