In this example, I have created a flextable, whose cells are populated by the data from a Spreadsheet.

function doGet(){

  var app = UiApp.createApplication();

  var panel = app.createVerticalPanel();

  var flexTable = app.createFlexTable().setStyleAttribute('border', '1px solid black')

      .setStyleAttribute('borderCollapse','collapse')

      .setBorderWidth(1); 

  //Get Data from spreadsheet

  var spreadsheetId = 't2Hx-ohk6E_DhtexDElvySA';//Change it to your Spreadsheet ID

  var dataArray = getData(spreadsheetId);

 //Load data into table cells 

  for (var row = 0; row<dataArray.length; row++){

    for (var col = 0; col<dataArray[row].length; col++){

      flexTable.setText(row, col, dataArray[row][col].toString());

    }

  }  

  panel.add(flexTable);

  app.add(panel);

  return app;

}

function getData(spreadsheetId){

 var ss = SpreadsheetApp.openById(spreadsheetId);

  var sheet = ss.getSheets()[0].getDataRange();

  return sheet.getValues();

}