Hi I installed Unity and Visual Studio way more before the course. So if any one have issue related to making visual studio as external editor and downloading visual studio 2017 tools for Unity here i will say how i did it.
please note before following the steps i say watch the final lecture video in section 2 of the course( 16 fixing visual studio problems)
- visual studio as external script editor
follow manual installation guide from link( docs. microsoft.com)
2)downloading visual studio 2017 tools for Unity
same link first paragraph on manual installation
hope this will help