If that article with the video is not working - its most likely because you have not setup the System user properly. You need to follow the article exactly as it and make sure you get the system user created, give correct permissions and use the Sling Mapping Service. Once you do all of this - it works as shown in the video.
If you are using AEM Forms 6.4, my recommendation would be to learn the Forms Data Integration  and integrate with a data model via JDBC using a mySQL database . That is fairly easy to set up and is a common pattern you would use with a real instance. Its also very common to offload form data via REST or SOAP but that is usually harder to setup for test purposes. You (should) never use JCR to store data in a real setup for many reasons. The JCR is great but its not recommended to use as a data storage method (mostly due to replication issues).
If you still want to use JCR to persist your form data, there are quite a few tutorials on how to read/write from the JCR in AEM