Here's a little introductory material. Still have a bunch of tools to talk about, but I figured, I might as well put out what I've written down so far.
Recommended tools:
XML Extractor (for extracting the XML game files in a way that makes them easy to search through for various files)
Link to tool: http://modthesims.info/d/563256
How to Use / What to use it for: Create a dedicated folder to use for the extracted game files. This will be your "Destination Folder." What I do is create a folder with a date as part of the name (ex: XMLExtraction3-23-2017). This way, I have a unique folder with the tuning files in it and I know what date it pertains to. You could name it based on the date you extracted or the patch date (the latter might be more clear in knowing when the files are from, in relation to the game's history).
Choose Full Automatic Extract. Game Folder should be aimed at your "The Sims 4" folder (mine is located under C:\Program Files (x86)\Origin Games\The Sims 4\). It'll probably be something like that. Check Include Strings from, Include XML References, Create Reference Files, and Create XML File Index. If you want the JAZZ files, you can check that. No reason I know of to check "Disable no tuning in package warning" - so leave that unchecked. Number of threads just has to do with how much of your CPU/ram will be devoted to extracting the files; choose what makes sense in relation to what your PC specs are and whether you're running a lot of other programs while extracting.
Under Output Options, choose TGI Only and (with everything else set) do Begin Extracting. When that's finished, choose Name Only and extract everything again. You don't strictly speaking need to do both. You could only do one or you could do TGI + Name. I do them separately, so that I have two different sets of files; one with the naming that would be recognized by a package file and one with the descriptive name (ex: motive_bladder). This makes it easier for me to locate stuff at a glance and easier to set up things like overrides.
Notepad++ (for editing .xml and .py files)
Now that you have a boatload of tuning files on hand, it's time to get a look at their insides and maybe make some tweaks. Notepad++ is a solid free editor for editing (and viewing) XML files.
Link to tool: https://notepad-plus-plus.org/download/v7.5.1.html
How to Use / What to use it for: There's not a lot you need to know about Notepad++, specifically. It's pretty much a standard text editor. There is one thing to watch out for and that's in relation to whitespace and indentations. As found here: https://stackoverflow.com/questions/3366499/notepad-indentation-messes-up
Go to Settings -> "Preferences..." -> Language Menu/Tab Settings and check 'Replace by space'
Go to View > Show Symbol > and choose Show Whitespace and Tab
Doing this will ensure that the spacing of your files edited in Notepad++ is recognized properly by other programs, such as S4PE.
If you're itching to get a look at some tuning files, you can go to the folder where you extracted files and open one of them up in Notepad++ (pick a file and right-click, Edit with Notepad++).
!Important!: So that you aren't saving over the game tuning files and thus cutting off your access to them in their original form (aka: making a mess of your workspace), if you plan on editing a game file, I highly recommend copy/pasting it into a different folder (for example, I have a folder called SimsModding and then I put project folders within it, like if I have a project called "BlarghMuffle," I'll put my BlarghMuffle project files in there). You can even go a step further and distingush whether you're doing an Override (overwriting an existing file) or making an original file, using a game file as a template.
For example, with Overrides, I'll usually have a dedicated folder named Overrides within my main project file and then if I have a file that overrides motive_bladder, I might make a folder within Overrides called motive_bladder and a folder within that called Original that holds the motive_bladder file in its original Maxis form, while my edited version of the file is contained within motive_bladder. Might sound convoluted and complicated at first, but it's a lifesaver to stay as organized as possible when you're working with a ton of files and trying to keep track of everything.