how do I navigate to forms mode?

Forms mode is only available in the desktop program, not in the online one. It  is located in the top left corner of the screen.

If using Online TurboTax, you can't see all of the forms during return preparation unless you pay in advance (we can tell you how.)    You won't be able to edit on the forms, however.    But you can see a few forms for free during return preparation.  You can preview the Form 1040 and Schedules 1-3.   Here's how:

With your return open, go to the left menu column and click on TAX TOOLS, then subtab TOOLS.

In the Tools Center click on "View Tax Summary."

On the Tax Summary screen go to left menu column and click on Preview My 1040.

That should open the Form 1040 Worksheet and Schedules 1-3.