1. Clone this repository
2. Run "getv8.bat"
3. Extract "v8-build-tools.zip" contents to your current directory
4. Run "buildv8.bat" from the command line with the the required parameters (e.g 'buildv8 Debug ia32')

Now you can open the solution and change code, build, etc...

There is a problem upgrading beyond tag 3.28.50 because 3.28.51 changed an API used by Javascript.NET. https://chromium.googlesource.com/v8/v8.git/+/3.28.51

If you want to upgrade furture then you need to replace the call with the new API equivalent.