Thanks to nbarnes for these awesome instructions!

Download Long War 1.0 (https://www.nexusmods.com/xcom/mods/88) and Long War Rebalance (https://www.nexusmods.com/xcom/mods/686).

Unzip and install Long War 1.0 using the instructions inside the repository.

Unzip Long War Rebalance.

Inside the LW Rebalance v1.* folder, you will find some documentation, a Linux install directory that we will be ignoring, and three folders; Config, CookedPCConsole, and Localization/INT.

Inside the Config folder, among the files, find:

DefaultMaps.ini
DefaultGameCore.ini
DefaultGameData.ini
DefaultContent.ini
DefaultLoadouts.ini
and copy them in place.

Take each copy and rename them like so (caps sensitive!):

DefaultMaps copy.ini - > XComMaps.ini
DefaultGameCore copy.ini -> XComGameCore.ini
DefaultGameData copy.ini -> XComGameGameData.ini
DefaultContent copy.ini -> XComGameContent.ini
DefaultLoadouts copy.ini  -> XComLoadouts.ini

You should now have a Config folder with the four copied-and-renamed files, along with the four original files and all the other files that we haven't touched.

Take all the contents of the Config folder and copy them to two places:

~/Library/Application Support/Steam/steamapps/common/XCom-Enemy-Unknown/XCOMData/XEW/XComGame/Config
~/Library/Application Support/Feral Interactive/XCOM Enemy Unknown/XEW/MacInit

Switch to the LW Rebalance 1.*/CookedPCConsole folder and copy all the files you find inside to:
~/Library/Application Support/Steam/steamapps/common/XCom-Enemy-Unknown/XCOMData/XEW/XComGame/CookedPCConsole

Switch to the LW Rebalance 1.*/Localization/INT folder and copy all the files you find inside to two different places:
~/Library/Application Support/Steam/steamapps/common/XCom-Enemy-Unknown/XCOMData/XEW/XComGame/Localization/INT
~/Library/Application Support/Steam/steamapps/common/XCom-Enemy-Unknown/XCOM Enemy Unknown.app/Contents/Resources/MacOverrides/XEW

Run XCOM. You should see a 'Long War' button in the lower left, replacing the 'Single Player' button. Clicking it should take you to the new campaign window with 'Long War Rebalance v1.xx.xx` at the top. If so, you've installed Long War Rebalance and are ready to get your violent xenophobia on!

Version
These instructions were written for Macintosh OSX 10.13.6 and Long War Rebalance v1.28.03 in September 2020. Visitors from the future, beware our fate, lest it be yours!

Notes and Troubleshooting
A. You may not see ~/Library at first; it is a hidden system folder located in your user folder. To show hidden files and folders, open Terminal and use these two commands

defaults write com.apple.Finder AppleShowAllFiles true

killall Finder

Now ~/Library should be right there for you.

B. To find /XCOM Enemy Unknown.app/Contents/Resources/MacOverrides/XEW, find ~/Library/Application Support/Steam/steamapps/common/. There should be an application icon for XCOM Enemy Unknown. Right click it and pick Show Package Contents. That will take you into the application's internal file structure, where you'll find /Contents/Resources/MacOverrides/XEW.

C. Note that for many of the destination folders, there is both a /XCom-Enemy-Unknown/XCOMData/XComGame/ and a /XCom-Enemy-Unknown/XCOMData/XEW/XComGame/ folder. See the /XEW stuck in there? Make sure you're dropping your mod files into the exact folder described above.