Yesterday I was trying to follow this guide but it didn't work. Even if it did work, it still lacks the function to redirect to custom thank you page after submission.
The easy way out is of course Google Form but it doesn't look professional. The formatting wouldn't follow the rest of the website and the color scheme is just garish.
After a quick search, I found FormSubmit.co which is super lightweight and easy. No registration needed and you just customize it like any other HTML form.
Best of all, you get the custom redirect after submission function for no added cost. There's no limit for number of submissions as well. The only catch is that all data is sent direct to your inbox but it's more than good enough to create landing pages for launching new projects.
Before this I used Ninja Form on WordPress, but this new method on Ghost is faster and cleaner.