Given...certain legislation that passed recently, I saw that re621 is one of the ways to mass download favorites. However, I'm illiterate in how to use it and I need it explained to me as simply as possible. I have no idea how to code and just followed the instructions to download tampermonkey and re621 for the appropriate browser (edge in my case). Could someone help please? Pictures would be much appreciated if feasible.

Well, if your re621 looks like my re621, you'll first select the select button underneath the tags on the left side of the screen.

You'll then scroll down until you've loaded every page of your favorites you want to download. It should load new pages by scrolling down as a setting automatically, mine did.
If not, you should have a new settings button in the top-right of the screen in between the bell icon (notifications) and the gear icon (e621 settings). It should be a wrench icon (re621 settings).
Go to General and look through the list for infinite scroll and make sure it's enabled. Now you can follow the first instruction of this section and load every image that is in your favorites on one page by scrolling down.

I recommend clicking your scroll wheel once. This will cause your mouse to scroll simply by positioning it above or below the spot you middle clicked.

Once every page of posts you want is loaded, go back to the top where you found the select button and click select all visible, then press download.

Make sure your computer does not power off during this time, otherwise you will have to start the process over, and the downloader does not go from top to bottom, so it will be hard on you if you find duplicates later on.
Try to keep the tab in focus, it seems to go faster that way.

If you have a lot of favorites, especially webm videos, it may pay to break them down into smaller groups by changing the tags you use to search for them.

Examples:
fav:Peskeon -webm -gif -flash for me results in a little over 6900 pieces. I'm not sure my computer got it all, since by the time I got back the tab was replaced with a crash message.

A better idea for me might have been say... fav:Peskeon -webm -gif human -humanoid -anthro -feral so that I only get posts that contain a human character (or just the human tag) and change what's included and discluded until I've gone through everything.
Also seriously, do get the webms separately. Save yourself some pain.

Edit: Besides all that, this gives me an insight into posts that aren't properly tagged. Posts that aren't tagged with human or anthro and whatnot.

Edit edit: No wait, if you press select first, it goes into manual loading mode. So don't do that first.

Thank you thank you thank you! Blessing upon you, your house, and your camel. Part of the problem was that since I never used tampermonkey before, I didn't enable the script to be able to run under tampermonkey's settings. Thank you for this help.

I started using Hydrus as mass downloader to save to a harddrive. It fetches everything from tags to notes. It's not exclusive to e621 too, it can get it from any booru.

I spent a good few months downloading about 10% of the site. This after the admin were elected but stopped after a few months. If I had kept it up I'm sure I'd have it all done by now.

So now, as it is, I'm going to just grab some favorite artists to put on batch, and any else I feel I should grab really. I can't get it all but there's probably some smarter people who can clone the site to a better country.