Domes over Plant Tray Cuttings
Completely air tight domes aren't really necessary.
Just a dome that will hold in moisture works just fine.
A common first timers mistake to rooting cuttings is to overuse the domes and leave them on for too long. This will rot out the cuttings with excessive moisture at the point where the stem goes into the starter plug.
If you ask 100 people how to root a cutting, you'll get 100 different answers!
There are a lot of good ways to do it, but based on the feedback from hundreds of customers over the years, my best tips for a first time cutting attempt would be to follow this schedule:
DAYS 1 - 3
Let the atmosphere get as humid as you can under the dome only for the first 2 - 3 days. For those first 2 - 3 days, you'll see droplets completely covering the inside of the dome.
DAYS 4 - 7
After that it's a slow progressive drying out of the atmosphere under the dome. For the next couple of days, you'll either open the vents on the top of the dome, or put the dome slightly ajar to let some air in. You want some condensation in 2 or 3 corners of the dome, but not completely covering it like it was for the first few days.
DAYS 9 - 12
Then, after that you'll completely pull the dome off for maybe an hour at a time a couple of times a day or so before you put it back on ajar.
After about 9-12 days of the gradual drying out of the atmosphere, the plant cuttings shouldn't need the dome at all.
Throughout the entire time that you're rooting the cuttings, you have to maintain consistent moisture in the starter plugs. They should never completely dry out.
This progressive drying out of the atmosphere around the cuttings will force them to grow roots while they're looking for water. If you leave the dome on for too long, it creates an artificially humid environment for them, and they don't need to grow roots. They remain in a form of suspended animation until they rot out.
Again, there are a ton of good ways to do cuttings, and this is just one of them. If you've got someone else giving you tips, try their way first. Hope this helps!