Open VS Code
Open Settings
Search for `Error Squiggles`
Set `C_Cpp: Error Squiggles` to `Enabled` for C/C++
Make sure the relevant language extension is installed
Check that the file type is recognized by VS Code
Save the file and wait for diagnostics to refresh
Reload VS Code if squiggles do not appear
Verify the language server or linter is running
Check that problems are not disabled in the editor settings