Starting Trichocereus cactus from seed is extremely simple- here is a basic outline of what you'll need to do:
1. get a container which has a transparent lid or use plastic wrap to cover your seed container. i personally like to use clear 5.5 oz portion cups with the snap lids.
2. fill your container anywhere from a quarter to a third of the way up with sifted soil- i use Fox Farms happy frog and sift out the larger chunks.
3. spritz the soil with water- this will help the seeds from bouncing around when you sprinkle them on the soil. Give the seeds another spritz- there should be enough moisture in the contaier that it creates a humid environment once the lid is sealed
4. place the sealed container on a window sill, on the porch, or inside under a basic light- nothing fancy is required to germinate them. as the seeds mature, they will appreciate more light. they can be left in the container for a few months. as they get to this point, you may choose to remove the lid and either transppant them to separate containers carefully or drill holes. once the lid is removed, they will need watering more often. Trichocereus are heavy feeders compared to other cacti.
notes: sterilizing the soil is optional. ive had great success without sterilizing the soil, however you may experience otherwise depending on the situation and soil composition.
make sure your soil doesnt dry out while waiting for the seeds to sprout. most should germinate within the first couple weeks.
if the seedlings start turning red, its likely they are receiving too much light- wane the light down until they recover the green color. it will not stunt the plant indefinitely but may take some time to bounce back from being burned.