Just to put into words what you want to see in the video - you want to select the larva that are both clear and "too small to see". I use a 7x magnifying loupe. These larva are about 80 hours old since they were laid as an egg by Mama Queen, that's about 8 hours after the chorion (egg shell) has dissolved. Any larva up to about 100 hours since egg-lay can make a queen, but most often, the best grafted queens are made from those between 6 and 10 hours after becoming a larva (78 to 82 hours since egg-lay).
Chinese grafting tool - Order about 5 of them and expect one to work considerably better than the others. The flexible "tongue" slides under the larva and blob of royal jelly. It flexes as it goes down into the "inverted pyramid" at the bottom of the wax cell, scooping under as much RJ as possible. Try to lift out the RJ + larva mass without touching the cell wall (I mentioned above that cutting the comb down to just above the grubs with a sharp knife helps out considerably. Try it!) Place the "tongue" of the grafting tool into the artificial queen cell cup from slightly off-center, that is, closer to the cell wall nearest you with the blob of RJ + larva facing away from you, touch bottom flexing the tongue over the center, and depress the button while sliding the tongue backwards across the center of the bottom of the cell cup. The larva should be face up, and still in the middle of the RJ.
Grafting hook - it is better to first prime the queen cell cup with RJ, then graft. Use a syringe when priming. Place the hook under the middle of the larva. Some RJ will make her a bit sticky, but don't rely on this. Balance her on the middle of the hook. Place her in the middle of the prime of RJ with a downward, plunging action, pulling the hook out backwards, not sweeping to the side. This way you are unlikely to flip her upside down.
Paint brush - The #000 artist's sable paint brush works a lot like the Chinese grafting tool, flexing as it goes under the larva and getting some royal jelly in the process, except you "roll" her off the brush sideways with about 1/4 turn of the brush. Perhaps the easiest of all the tools to graft with, and you could prime the Queen cell cups with RJ as well.
Try them all - everybody is different. Larva CAN and DO dry out in the time it takes to graft 32 to 48 queen cell cups. Cover them with a warm, damp cloth as you go, and keep a spray bottle of warm, purified water handy. Do one bar at a time, placing the frame back into the Cell Starter colony after each bar is grafted. Graft the next QC bar, take out the QC frame and add the newly grafted bar, float it down into the slot next to the pollen. Graft the 3rd bar (if using a deep) in the same manner, pull the QC frame out, add the 3rd bar, float it down into the slot, close the hive up and fill the feeder jar.
BTW - If you are priming your queen cell cups with a syringe of royal jelly, the composition of royal jelly changes from Day One to Day 5 of a queen larva's feeding. More sugars are used early on, and more protein is added later. GET THE ROYAL JELLY FROM ONE-DAY-OLD QUEEN CELLS!