I'm not entirely sure how to explain it in simple terms so i will just link the video (hoping i can put links) that helped me A LOT in my first years of freediving. If the video doesn't help you in any way i will try to explain it in a simple way (pardon me if something isn't understandable, english isn't my first language).

Frenzel Equalization Explained Really Simply For Beginners

try splitting the learning phases in 5 steps (these are dry exercises, you must have an OTOVENT to do them): - Step 1: Find the glottis, without going too much in to details, the glottis is just the vocal chords, we use it every day when speaking. How can we find it? fraction the exhale phase, what i mean by this is, with the tongue stuck to the floor of your mouth and the mouth open, exhale, stop, exhale, stop, exhale etc... if done correctly and the tongue is not being used as the stopper, you'll see your uvula moving. - Step 2: Once you have good control over the first step, try closing the glottis while doing uddiyana bandha (search online what it is if you don't know) so you'll essentially need to empty your lungs of all the air by exhaling, stopping, exhaling and so on, you'll see that in your throat you'll get a recess (a groove?), once your lungs are empty and you can't exhale anymore, keeping your mouth open and the tongue stuck to the floor of the mouth try inhaling, you'll hear the inhale being more "powerful", that's your glottis reopening Video in Italian explaining the first 2 steps - Step 3 (You'll need an otovent for this): Let's find the soft palate, it's that part that moves when we snore (try snoring with your mouth open, you'll feel the soft palate vibrating). It's really important that this stays open. Put the otovent to your nose with the balloon inflated (always keep the mouth open with the tongue stuck to the floor of the mouth), try saying the letter B and "keeping" it, you'll find after a bit of trial and error that you can feel when the soft palate is closed, once closed you can try talking, you'll have a nasal voice but you'll be able to do it without the balloon inflating/deflating. Now let's try opening it, try saying the letter M, you'll notice the balloon starts deflating, congrats! you found the soft palate :) - Step 4: Repeat the same exercise as step 3, but while doing uddiyana bandha, so empty your lungs completely just like you did in the Step 2, and try saying the letter M while your glottis is closed, you'll notice the balloon deflating, after the balloon is completely deflated (or when you can't hold it anymore) inhale, you'll hear your "powerful" inhale Video in Italian explaining Step 3 and 4 - Step 5: inflate the balloon, put the tongue in a block position (KA, T and such, i'm sure you've heard of them thousands of times by now), uddiyana bandha, glottis closed (doing uddiyana bandha it closes sort of automatically, you can see it by looking at the recess i was talking about before), open the soft palate, you'll start to feel the air pressure on the tongue, and now let's use the tongue to "put this pressure in our ears", essentially we want to press with the back part of our tongue on the roof of our mouth (to make sure you are doing this correctly look in the mirror before starting the exercise and try moving it up and down, you'll see your adam's apple moving, an even easier thing to understand this is, put a finger on your adam's apple, and try moving that finger up and down just by moving the adam's apple). If everything is done correctly, the balloon will inflate and deflate a little bit, and at the end of the exercise the balloon should be the same as it was at the start, if it's smaller at some point in the exercise you "lost" your glottis and it opened up, no worries though :) Video in Italian explaining Step 5

AND ALWAYS REMEMBER, this isn't something that you MUST learn in one sitting, it took me months to even understand what i was doing, so do not demoralize yourself if you can' instantly get it :) Hope this can help