haha, I looked because I was going to put the graphs on the nand, but now I know there will only be about 20meg free space, I think I will either put on a usb stick on my second openwrt device or use a network mount.
I do have two openwrt devices, so its looking more likely I will just move the C7 to the second device and keep that hosting all the graphs. (it currently hosts for the second device as well). The second device is a second ethernet switch and a local 5ghz AP for my second room. (currently using a plusnet one hub).
On that page, click on "Customize installed packages ..." then after all the listed packages add " luci" (including the leading space) to add luci to the image. I would also recommend adding " luci-app-attendedsysupgrade" (including the leading space). Then click "REQUEST BUILD" and wait a few moments and your package list will be checked, then the image will be built and the download links will be a newly built "factory" and/or "sysupgrade" image including the Luci web interface. Download the factory image and upload it to the router from the stock firmware webgui and install.
If the stock firmware complains about it being an incorrect firmware version, just click continue or similar and it should load OK and reboot to OpenWRT at 192.168.1.1 after a few minutes.
For later upgrades, the second added package I recommended above also adds another item in the system menu, attended sysupgrade, that allows you to upgrade the firmware later straight from the webgui with a newly built version with the same installed packages. Or if you don't want to do that you can go back to the firmware builder page and add wanted packages and download a sysupgrade package to load from Luci.
Just did a sysupgrade (using the luci-app-attendedsysupgrade option in the system menu) to SNAPSHOT - r22788-0ce356bbba and everything is working great!
I've been using snapshots since February and apart from two bugs that are now fixed relating to the 2.5G WAN port, everything has been pretty much flawless. The only thing it makes a little difficult is adding in packages that depend on stable builds with a certain kernel version or that are only in the stable package repository.
The 23.X release will probably come in the next month or so for stable builds as I saw Ansuel (one of the key developers) say elsewhere in the forums that he intended to be working on branching 23.x from master this month but had been a bit overrun.
If you are coming from the stock Netgear firmware you need a factory image to upload from the Netgear webgui (or other methods in the wiki).
If you have already installed a recent OpenWRT factory image you generally only need a sysupgrade image, but there should be no harm installing a factory image.
If you already have OpenWRT installed and if you installed the Luci-app-attendedsysupgrade package, there will be an extra item in the system menu called "Attended Sysupgrade". If you use this, by default it will not save any file on your computer, it just uploads straight to the router and upgrades. You can choose to save the file during this process after it builds successfully and before you choose to progress to upload/install.
The attendedsysupgrade process uses the same build process as the firmware builder web page but it's just for convenience. The file it generates is only a sysupgrade image (usually with any packages you had installed like Luci added in to the image already) as you should not normally need a factory image if you are using attended sysupgrade. It will show you the list of packages to be included with the image and allow you to add or remove any before building, but it only offers a sysupgrade image.
You could get either a factory or sysupgrade image by going to the firmware builder page and choosing to add whatever packages you want via the customise option. I would recommend adding in Luci as otherwise the snapshots don't have it, only stable builds have Luci built in by default and otherwise on snapshots you will be stuck using command line tools to install and upgrade. You can have a factory image with only Luci added and install over this with a sysupgrade with other packages installed, so a factory image does not need to be upgraded with a newer factory image unless the first was quite old, so in most cases you should be able to just use a sysupgrade over a factory image from 3 or probably even 6 months ago as long as no breaking changes (perhaps like the 21.x -> 22.x firewall changes) have happened in the meantime.