The easiest way is to use the Form Manager button within the post / page editor.  Look for the button towards the right of the editor:

Many thanks to Andrea Bersi for implementing the editor button.

 

Alternatively, you can type in the shortcode for forms directly.  The shortcode format is simple:

(form '(slug)' not found)

So if your form’s slug is ‘my-contact-form’, the shortcode would be:

(form 'my-contact-form' not found)