まずは、makemigrationを行う

エラー①が出た・・・

ERRORS:
auth.User.groups: (fields.E304) Reverse accessor 'Group.user_set' for 'auth.User.groups' clashes with reverse accessor for 'users.MyUser.groups'.
	HINT: Add or change a related_name argument to the definition for 'auth.User.groups' or 'users.MyUser.groups'.
auth.User.user_permissions: (fields.E304) Reverse accessor 'Permission.user_set' for 'auth.User.user_permissions' clashes with reverse accessor for 'users.MyUser.user_permissions'.
	HINT: Add or change a related_name argument to the definition for 'auth.User.user_permissions' or 'users.MyUser.user_permissions'.
users.MyUser.groups: (fields.E304) Reverse accessor 'Group.user_set' for 'users.MyUser.groups' clashes with reverse accessor for 'auth.User.groups'.
	HINT: Add or change a related_name argument to the definition for 'users.MyUser.groups' or 'auth.User.groups'.
users.MyUser.image: (fields.E210) Cannot use ImageField because Pillow is not installed.
	HINT: Get Pillow at https://pypi.org/project/Pillow/ or run command "python -m pip install Pillow".
users.MyUser.user_permissions: (fields.E304) Reverse accessor 'Permission.user_set' for 'users.MyUser.user_permissions' clashes with reverse accessor for 'auth.User.user_permissions'.
	HINT: Add or change a related_name argument to the definition for 'users.MyUser.user_permissions' or 'auth.User.user_permissions'.

解決策①

この１文を入れていないのが問題だった。

AUTH_USER_MODEL = 'users.MyUser'

入れたらOK！

気を取り直して、makemigrationだ！

またエラー②・・・

ERRORS:
users.MyUser.image: (fields.E210) Cannot use ImageField because Pillow is not installed.
	HINT: Get Pillow at https://pypi.org/project/Pillow/ or run command "python -m pip install Pillow".