Vermiculite is the best. Here's how.
Start with dry vermiculite and slowly add tepid water while mixing with a spoon or stick.
Every now and then, grab a clump in your hand and squeeze it into a "snowball"
Squeeze it as hard as you can.
It should stay in a ball, and make your hand moist but it shouldn't drip water.
If it drips, add more dry vermiculite until you get the consistancy correct.
That way has worked for me for 20 plus years.. It will work for you too
Perlite also works as an egg medium, but it is very hard to tell when it's wet or dry as it looks the same. It also can't be sqeezed, so getting the water ratio correct is more difficult.
I prefer long strand Sphagnum over perlite, but vermiculite is the way to go.. it's cheap, easy to get and easy to get the water content correct using my "squeeze technique" above.
Happy hatching