Disable a button in ViewModel

Original I have code event in code behind.

private async void Start_Click(object sender, RoutedEventArgs e)
{
     try
     {
         Start.IsEnabled = false;
         Start.IsHitTestVisible = false;
         Start.Background = Brushes.Red;

Now I want to move code to ViewModel as MVVW pattern.

<Button Grid.Column="1" Grid.Row="3" Command="{Binding StartProcess}" Width="200">Start</Button>

using GalaSoft.MvvmLight.Command;
public RelayCommand StartProcess { get; set; }
public MainWindowViewModel()
{
    StopProcess = new RelayCommand(StopProcessExecute);
    // etc.
}
private void StartProcessExecute()
{
     // how to disable the button.
     // blah blah
}