The soil mix is very controversial. I like a combination of native loam soil at about 80% mixed with 20% sphagnum. Add some grit if desired.
I prefer sowing 1 kind per 1 pot, and not to sow many kinds in a common flat, as they can have very different watering and temperature needs.
Scatter the seeds on your soil surface, and then sprinkle a tiny bit more soil over the seeds to barely cover them. Then over this I like to cover the top surface with a thin layer of sandblasting grit. The grit we use is about 1 mm in diameter and I prefer to use a dark opaque color.
Put the pots into a shallow box for germination. You can use spare household equipment from the kitchen or build some from wood. I put side walls about 8 inches high on a sheet of plywood. Over this box I like to cover it with nylon window screen that is available at most lumber or hardware stores. During germination the cover remains on the box, you can mist through the screen and light and air passes readily. For the first several days you can mist several times daily to keep the pots moist. After a week the number of times to mist can be reduced to once or twice daily. Once the pots start to sprout you can remove them from this area and place them where there is not as much misting or humidity.
You can put the box in your greenhouse, or on a sunny windowsill area, or indoors under lights. I do a lot of germination with the boxes outdoors under a layer of shade cloth. The natural sunlight and heating is wonderful, and rainfall is the best germinator of all. Naturally if heavy rain is expected precautions will need to be taken. Make sure your box has plenty of drain holes so water can not pond.
The above methods are fine for Sclerocactus and Pediocactus with a few changes. These seeds need a long period of exposure to freezing and thawing conditions. Nothing has improved upon sowing the pots outdoors during the winter in a temperate climate. I try to get the seeds sown by mid January. They just go outdoors in a box exposed fully to snow, cold winds, and everything else that happens in the winter. It does get cold here in the winter and we can have 2 or 3 feet of snow. Quite a few nights fall to between 0F and 10F with a few nights below zero F. Then in early spring I start to water them daily. It is still freezing a bit at night and warming up in the day to 50 to 60F. Water them daily all spring, they are really quite thirsty at this time. During the heat of summer water less, just give an occasional heavy soaking. Most germination takes place in the spring or early summer. In the fall cool and dry.