Using Captcha with Contact Form 7

I’ve installed Contact Form 7, which works very well
I want to use Captcha to make the email less vulnerable to spam, so have downloaded and activated ‘Really Easy Captcha’ (recommeded for CF 7)

This may sound dumb, but how do you actually get the Captcha to work?
Is adding the tags sufficient, or do you need to edit the plugin code?
I cannot make much sense of the installation intructions on the CF7 or ‘Really Easy Captcha’ sites

I’m something of a beginner with WordPress