I start by copying things such as my lengthy DNS options, VPN certs/keys/options, and firewall and startup commands to a text file. Those ‘big blocks’ are easy to copy out of the webpage, and you can just as easily copy and paste them back in again later.
Then I use Snagit to make a video, and I go through all of the tabs and scroll up and down while it’s recording. You can go quite fast as you pause the video when playing it back later.
I used to do screenshots but I find video to be much faster to make and easier to refer to later.
Edit: remember to expand all sections, for example the advanced wireless options, to be sure you’ve captured everything in your video.
I like the screenshot method using Firefox. Once or twice a year or if I make significant changes I take a screenshot of every setup page and save to a "Setup" folder.
Make sure each page is fully extended (Advanced Settings = checked, etc) Right click the page, select "Take a Screenshot", select "Save Full Page" (upper right), click the Download button when it appears (can take a few seconds) and select a folder to save images in. Lets me open each page in a viewer while having UI open to check/change settings changing back and forth as needed.
Edit: I name each screenshot the name of the UI tab which allows me to save only pages I have edited without the need to redo ALL tab shots. I have moved old setting shots to a second folder JUST IN CASE I want to revert to the previous settings. In a day or two, if I'm happy with the new settings I can either delete the old shots or put them in a dated folder for future reference or reversion.
All shots in my DD-WRT Setup folder are always my current settings.