If I find any errors, or anything to add, I'll try to remember to
keep this as a self-contained set of instructions, by always repeating the FULL SET of instructions in any further posts, with the amendments included.

I invite anyone else to make corrections/improvements in the same way.

Make the Simply Fortran version of gfortran etc
accessible as terminal commands
   Add this line to the $HOME/.profile file:
       PATH=${PATH}:"/Applications/Simply Fortran.app/Contents/MacOS/" 

As explained in the Dislin README file
   Install OpenMotif in .Applications.
   In Applications/OpenMotif21/lib,
   create the symbolic link libXm.dylib to libXm.3.1.dylib:

ln -s   /Applications/OpenMotif21/lib/libXm.3.1.dylib  /Applications/OpenMotif21/lib/libXm.dylib

   
   Install Dislin in $HOME/{rest-of-your-chosen-pathname}/dislin,

   Enter these instructions in the $HOME/.profile file:
       DISLIN={dislin directory path]                     
       export DISLIN
       PATH=${PATH}:${DISLIN}/bin

       DYLD_LIBRARY_PATH=$DISLIN:$DYLD_LIBRARY_PATH
       export DYLD_LIBRARY_PATH

Re-compile the applicable dislin.mod file
with SF’s current version of gfortran

cd {pathname to}/dislin/gf/
   gfortran -c -o dislin.o dislin.f90

Use these settings in Simply Fortran project options
   General:
      Target name:  /Applications/{Program-name}.app
   Code generation:
      Target cpu: Generic
      Memory model: Default(small)
   Compiler flags:
      Linker: 
        -L/Users/John/DEV/dislin  -ldislin  -L/Applications/OpenMotif21/lib  -lXm
   File locations :
      Modules/Include files:  {path to}dislin/gf
      Libraries : {path to}/dislin
     

Additional step needed to resolve runtime errors

   With successfully compiled and linked executables
   these runtime errors may occur:
    Dyld: Library not loaded: /usr/local/lib/libdislin.11.dylib.
    Referenced from:  /users/John/DEV/InStruct/Mac.SF/InStruct64
   and/or
    Warning: Widget must be a VendorShell
    >>>> Font cannot be loaded! (Helvetica            )
    Error: attempt to add non-widget child “dsm” to parent “dislin” which supports only widgets.

   Solution:

   In system folder, /usr/local/lib/,
   create the symbolic link libdislin.11.dylib  to  libdislin.11.3.0.dylib

sudo ln -s {full path to dislin}/lib/libdislin.11.3.0.dylib  /usr/local/lib/libdislin.11.dylib

   Set the new link’s permissions to.  rwx  r-x  r-x

sudo chmod 755 /usr/local/lib/libdislin.11.dylib

   There are two versions in system folders of the same dynamic library libXt.6.dylib :
   (a)  /opt/X11/lib/libXt.6.dylib   (699 kB)
   (b)  /opt/X11/lib/flat_namespace/libXt.6.dylib   (718 kB)

   Made a backup copy of version (a), then make MacOS use version (b)

sudo mv /opt/X11/lib/libXt.6.dylib /opt/X11/lib/libXt.6.dylib.org
   sudo cp  /opt/X11/lib/flat_namespace/libXt.6.dylib  /opt/X11/lib/libXt.6.dylib