From 0.4.15 creating subcircuits is even easier.
You don't need to create a .subcircuit file, the .simu file is the subcircuit file.
So you have to save that circuit and move it to the corresponding folder as you did with the .subcircuit file before.
This has the advantage that you can easily edit the subcircuit: just open it in simulide, edit and save.
The tutorial has been recently updated (but still need some work):https://www.simulide.com/p/new-subcircuits.html
You can create the subcircuit and packages in the same circuit, so you can see and/or edit the subcircuit and the package easily:
Having a look at existing subcircuits can also help.
Check that your circuit works as you expect before creating a subcircuit.
As extra information:
You can now also create boards and shields for Arduino or other boards.
This is still experimental and there ase some problems to solve.
There is a very brief tutorial and video here:https://www.simulide.com/p/boards.html