So, this is our first exercise, not counting the preparatory one you just did, singing all up and down the neck in your chosen interval.
As mentioned in the video, I recommend that you start out with one of the following intervals b3, 3, 4, 5, because these intervals will be most readily available to most of you, since you have heard them so many times.
Use your deck of cards, and if you are at all unsure about the theoretical spelling of the intervals you are about to practice, review the spellings again, since you need to know not just the regular spellings, but also the double sharps and double flats, and odd notes like Cb or E#.
Then if you get the card C, listen to that C, then sing that C, and then sing say a perfect fourth from C, which would be F upwards, and G downwards. Listen and correct yourself if you got it wrong. And, again, don’t worry if your voice is wavering, this is most likely not because of your ear, but because you are not a trained singer. As long as you are roughly in pitch, that is OK. The important thing is to acknowledge all the sounds by EAR, unless you are a singer, it is not as important to sing them professionally. What you do need to do though IS to sing as well as you can, since that will greatly help your ear internalize all these intervalls.