Now that you have the ISO image of YunoHost, you have to put it on a physical medium: USB key or CD/DVD
Download UnetBootin for your operating system and install it.
Put your USB key on, select your YunoHost ISO and click "OK"
The software to use differs regarding your operating system.
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On Windows, use ImgBurn to write the image file on the disc
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On Mac OS, use Disk Utility
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On GNU/Linux, you have plenty of choices, like Brasero or K3b