After you have installed and activated both plugins, open the Contact Forms page and then Edit your contact form.
You will have to make a minor change to your contact form markup.
There are 3 options that you can use, one for author name, email and URL.
akismet:author akismet:author_email akismet:author_url
Add these options to their corresponding fields.
If you don’t have any form fields for an URL, then you can skip this option.
If you have multiple forms, you will have to repeat this process for all.
And that’s all it takes. Now every form submission will go through Akismet spam filtering. For that to happen, some data will be sent to Akismet servers for processing. This happens almost instantly, but you should be aware of this data transfer.
If you are worried about GDPR compliance, you can read about Akismet and the GDPR on the official Akismet website.
Like after any change to your contact forms, I recommend that you make a few test submissions to make sure that everything works correctly.
I also have a tutorial on How to Save Contact Form 7 Submissions to the Database.