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Fill a pot with a moist, cacti compost. Gently firm down and level the surface. Scatter your mixed cactus seeds over the surface, taking care not to sow them too thickly.
Gently sprinkle a thin layer of vermiculite or fine grit over the seeds, covering the whole surface of the compost. Leave the pot in a greenhouse or on a warm windowsill, covered with a clear plastic bag to preserve soil moisture.
Seedlings will develop within a few weeks. Remove the plastic bag and water when the compost becomes dry, but spray the surface with water regularly, to keep it moist.

Coir discs are compressed coir that expand to the exact size needed for easy seeding, rooting or growing in aeroponics or hydroponic systems. They can make coco coir discs wrapped with a sleeve or without. Great for seed sprouting in mini greenhouses, bonsai, micro greens and more.

You would add these discs to a pot or container and add water. They will began to expand and when the all the water has been absorbed you can plant the seeds that came in your kit.

Dome lids are a convenience, not a necessity. It helps in retaining moisture; if seeds dry out at a critical time, it will not germinate. If you check your starts twice a day and spray them with water when they look dry on top, your seeds will germinate. Of course you can cover the pots with plastic wrap, put them into plastic bags (I like to cut a couple of air holes) or flip a large food storage container over them. Remove the plastic wrap every few days to let air in and remove it completely when germination starts; otherwise the seedling may develop fungus. Keep your pots in a warm place but not in direct sunshine until germination begins and the domes are off.

It is a bit early. It would be hard to keep them from getting leggy unless you have horticultural grow lights for them. This article gives you info on seed starting in zone 8.

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-8/zone-8-seed-starting.htm

Some seeds can be started indoors in March and April, and transplanted in May or June.

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-6/plants-for-zone-6.htm

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-6/zone-6-flowers.htm

I am always suspicious when someone receives seeds in the mail without growing instructions. I looked up the plant and couldn't find any seeds or plants offered from reputable nurseries. They were offered online from individuals touting "rare" find. The reviews were all negative.
Generally, seeds can be sown in seed-starting potting mix, barely covered with mix, misted well, then placed in a plastic bag to create a humid environment. When seeds sprout, remove the plastic. When large enough to handle move the seedlings to individual pots.