Important Note: You have to go through the upgrade process before doing a clean install. That makes your valid Windows 7 or 8.1 key associate with your Windows Account. If you don't, you will need to call Microsoft, and everyone know how horrible that is! Here's what Microsoft says:
Once you've upgraded to Windows 10 using the free upgrade offer, you will be able to reinstall, including a clean install, on the same device. You won't need a product key for re-activations on the same hardware. If you make a meaningful change to your hardware, you may need to contact customer support to help with activation. You'll also be able to create your own installation media like a USB drive or DVD, and use that to upgrade your device or reinstall after you've upgraded.
Downloading Windows 10 ISO
To download the image file, you'll need to go Microsoft's website.
From there, you can choose to download either a 32-bit or 64-bit version of the Windows 10 Download Tool. Before you do that; you might want to review if your PC can handle Windows 10 (If you are running Windows 8.1 or a modern PC with Windows 7 SP 1 you're probably ready to go, but hey - it's best to double check everything first).
What do I do next?
You now have three options for installing Windows 10. Unlike the upgrade, you can't do it without using external media. This version, like Windows 8.1 ISOs, include a Media Creation tool. Run it and start the install wizard.
You can either select to Upgrade this PC now - which is interesting since you don't have to wait on Microsoft to do it if you reserved a copy.
Or, you can select to Create installation media for another PC. If you do that, you will be able to use a DVD or USB drive to install the media. It works a lot like the Windows 8.1 version did, or say a the free utility Rufus.
When you use the option to create installation media, select your language, edition (Home or Pro) and the Architecture (32 or 64-bit).
After that, you need to choose the media you want to use. You can either use a USB flash drive that's at least 4 GB or download the ISO and burn it to a DVD later. The good news if you want to create a DVD is that Windows 7 and Windows 8.1 both have ISO burning utilities built-in.