Step by step Sharepoint 2010 Modal Dialog Pop up

<script type="text/javascript">

     function OpenDialog(URL) {

         var NewPopUp = SP.UI.$create_DialogOptions();

         NewPopUp.url = URL;

         NewPopUp.width = 700;

         NewPopUp.height = 350;

         SP.UI.ModalDialog.showModalDialog(NewPopUp);

     }

 </script>

Call the Javascript on Button click and the modal PopUp will pops up with the Page of Specified URL.

btnOpenPopUp.Attributes.Add("onClick", "javascript:OpenDialog(‘/_layouts/CustomPage/CustomPage.aspx’);");