You capture the Heart Rate Variability (HRV) measurement as expressed in RR intervals by using a sensor and having it read its output to a software application with which you can see a display of the data or download it for viewing and manipulation.
To get your HRV readings from your body you can use a heart rate belt or other type of sensor and read the output on an app. To actually play with the data you will have to find an app that downloads the data in a format such as .csv or .txt that can be ported to a program like excel or R.
To learn how HRV works, I highly recommend you start by looking at the RR intervals so you can get familiar with the source data. Using apps that have already derived some measure is easier once you understand the underlying mechanics of the RR intervals. I wasted a lot of time trying to understand the apps before the underlying mechanics. You can avoid that.
Find out more by reading about Sensors.