At first, this is not a method - this is a property.
All you have to do is the following:
<ComboBox x:Name="ddlMonth" Grid.Row="3" Grid.Column="1" ItemsSource="{Binding Months}"/>
The trick is to make sure that any class that has the property Monthsis set DataContextfor either ComboBox or one of its parents (including the page).
So, if a property Monthsappears on the same page that hosts the ComboBox, you can add this line of code to the constructor or event Loadedon the page:
this.DataContext = this;
Months ViewModel ( , !), ViewModel DataContext ( , () ComboBox).