Try just holding firmly above the vent, don't press hard with your thumb like in the vid. Then slide your other thumb gently but firmly and evenly up the tail, starting halfway down it, towards the vent. The hemipenes lay inside the tail like small slim balloons and can go down 12 or 14 scales from the vent. You have to push them inside out to see them but if you press down hard above the vent, or start pushing the tail nearer than 14 scales down from the vent you could be blocking them coming out
I'd recommend watching a few more popping vids on YouTube if you're not sure but good luck and with practice, it gets much easier over time.