In Classic TWS or Mosaic, right click on any option or future option symbol in the Financial Instrument field and left click on the "Strategy" button. This will open the window to build an order for a Complex Position. In the upper left portion of the window you will have a choice to display "Calls and Puts" to build your own spread or display predefined Complex Positions. In the drop down menu will be choices for predefined "Horizontal Spread" (Calendar), "Vertical Spread" or "Diagonal Spread".
If you select "Calls and Puts" the option chains will populate in the top half of the window and you can select the individual options to build your option combo. Click on the options you wish to add, and they will populate in the Strategy Builder towards the bottom half of the window. If you wish to choose "Horizontal Spread" (Calendar), "Vertical Spread" or "Diagonal Spread", you can click on the menu icon in the top right to edit the spreads you want to display. Click on any predefined complex position strategy and it will display in the Strategy Builder portion window. You may then edit/check the order details before submitting.
More information on the Strategy Builder can be found in the TWS Users' Guide.