diff --git a/macos/README.txt b/macos/README.txt
index 63227dce8..742515b6a 100644
--- a/macos/README.txt
+++ b/macos/README.txt
@@ -2,21 +2,21 @@ WHAT IS THIS
 ------------
 This document describes how to build Tux Paint for macOS 10.12 Sierra and later.
 
-Tux Paint 0.9.22 and earlier required building Tux Paint from the XCode IDE.
+Tux Paint 0.9.22 and earlier required building Tux Paint from the Xcode IDE.
 Starting with 0.9.23, however, Tux Paint for macOS is built as though it were a
 Linux application.
 
 
 PREREQUISITES
 -------------
-Although Tux Paint is built without the XCode IDE, XCode itself is still required
+Although Tux Paint is built without the Xcode IDE, Xcode itself is still required
 to build Tux Paint.  Download it from the App Store, and launch it once to
-accept its license agreements.  Also install XCode command line tools using the
-command:
+accept its license agreements.  You may also need to install the Xcode command
+line tools using the command:
 
   xcode-select --install
 
-Building Tux Paint also requires various packages.  We install them from
+Building Tux Paint also requires various libraries.  We install them from
 MacPorts where possible, source code otherwise.  Install MacPorts to the
 default /opt/local path according to the instructions found on their website:
 
@@ -61,7 +61,7 @@ Tux Paint.
   *** WARNING ***
   ---------------
   Having any UNIX-like toolset installed on your Mac besides MacPorts and
-  XCode, such as Fink or Brew, will prevent your app bundle from being
+  Xcode, such as Fink or Brew, will prevent your app bundle from being
   portable.  Be sure Fink and Brew are not accessible from your build
   environment.
 
@@ -85,6 +85,53 @@ KNOWN ISSUES
   this writing we know Tux Paint cannot be built to run on macOS 10.7 or
   earlier.
 
+  See "OLD VERSIONS OF MACOS" below for best-effort instructions on how to
+  obtain, install, and build Tux Paint on an old version of macOS.
 
-August 2, 2020
+
+OLD VERSIONS OF MACOS
+---------------------
+Some old versions of macOS can be downloaded from Apple's support page:
+
+  https://support.apple.com/en-us/HT211683
+
+macOS does allow dual booting of multiple versions of the OS, but it's safer
+and easier to install the old macOS onto a flash drive.  But wherever you're
+installing it, the target drive's partitioniong scheme and partition type must
+match what the old macOS expects, so use the Disk Utility to partition and
+format the flash drive accordingly.
+
+As of this writing, the oldest version of macOS available on Apple's support
+site is Yosemite 10.10, which expects GPT (GUID Partition Table) partitioning
+scheme instead of the older MBR scheme, and `Mac OS Extended (Journaled)` as
+the partition type instead of the newer APFS partition type.
+
+Upon launching the installer, if you get a popup about macOS being too old or
+new to be installed, a bootable installer can be created using the instructions
+found here:
+
+  https://support.apple.com/en-mide/HT201372
+
+I found that macOS can be installed onto the bootable media itself, so you can
+make the flash drive into a bootable installer then install the old macOS onto
+the same flash drive.
+
+Once the old macOS is installed, you may find the Xcode on the App Store is too
+new to run on the version of the old macOS.  Old versions of Xcode can be
+downloaded from Apple's Developer site in an area accessible with free
+registration:
+
+  https://developer.apple.com/download/more/
+
+The list of macOS versions and the last version of Xcode compatible with them
+are laid out nicely on the Wikipedia page on Xcode:
+
+  https://en.wikipedia.org/wiki/Xcode
+
+And because Xcode is being installed manually, you can skip the step to install the Xcode
+command line tools (do not run `xcode-select --install`) but otherwise build Tux Paint
+using the same steps described in the earlier part of this document.
+
+
+January 9, 2021
 Mark K. Kim <markuskimius@gmail.com>