I think they are explaining how to grow 'starts' so that one doesn't have to plant seed directly into the garden and have germination issues/loss.
There are two kinds of seeds: one that should only be planted directly into the garden soil (peas, spinach, potatoes, leafy vegetables, carrots) and the other are more tender plants that should be started in small pots in potting soil under artificial lighting then acclimated to the out of doors garden and sun before transplanted into the garden (broccoli, cauliflower, peppers, tomatoes, egg plant, squash, cucumbers).
Update: 40 X 20 X 20 centimeters? So about 20 inches by 10 inches by 10 inches? Makes a difference. You are planting in a pot. You have to go by pot rules and if you bought soil in a bag from a store you did the right thing. You need to add drain holes. Plastic will protect the wood if not pressure treated, or spray 'Rhino Liner' on the wood to seal and protect the wood from moisture.
This had to have holes at the bottom and the bottom needs to be raised off of the patio or deck so that it doesn't sit in water. After the first few waterings the soil won't be coming out of the holes. The holes should be a good inch in diameter; at least 4 offset down the length.
Do not amend your potting soil at all. Just fill your planter/pot firming the soil with your fist, not pounding but firming to get rid of big air pockets. Do not add gravel or rock below the soil and above the holes. You should have at least one solid inch below the rim of your planter and the finished soil surface for proper watering.
When you have planted your starts (recommended), if you've grown your own starts, it is then you need to add a little fertilizer. Osmocote 14-14-14 is the easiest and safest and should last the entire season. Less is Best, More is Death and None is Dumb. Does this dumb ditty help at all?
If you are planting store bought starts please ask the store when the last time the starts were fertilized and what light are they used to: out of doors sun or artificial lighting? If you found them indoors, ask how long the plants have been indoors and out of the nursery.
Seeds are best started as per the diagrams on your bag. Little tiny pots for little tiny seeds. I use a spray bottle to water until the plants get large enough to have roots to suck up the water. Or just that little bit of soil, too wet, will damp off your seeds and baby plants. Rot. I'd up pot once, into 3" pots with potting soil, allow to grow roots and then transplant into your planter. Water the soil well around your transplanted starts. When they settle in you'll need to add a bit more fertilizer. Use half of what they recommend. Smaller applications is better than one big one. Less is Best. Osmocote is as simple and safe as you can find.
I should warn you that you aren't going to grow a lot of plant in your planter. One basil (Thai Basil if you can find it) will take up half of that planter easily, just a cute tidy round little plant. Yummy. You might have room for 2 peppers. Thai peppers are wonderful, again a round, tidy plant with smaller leaves and upright peppers, little yummy wonderful red peppers that point straight up and are profuse. One basil and one pepper. I grew them in pots; 3 gallon pots...about half the size of your planter. So one Thai basil or two european basil, and one Thai Pepper or two cayenne/jalapeno? Don't get carried away unless you've got extra soil and pots with drain holes. Grow your starts out of doors. If you do this in doors and want to put your planter out of doors in the sun, you have to acclimate the plants used to the indoors, the light coming from the kitchen window? to be able to handle the real sunlight. Basil can be grown fairly well in a southern facing sink window sill but it won't be the incredible Thai basil plant and who knows how it affects the flavor? Do you have a patio? You most certainly are able to grow lots of food in pots, with potting soil, drainage, a little fert, and sunshine...or artificial lighting. Plants have to have the right amount of light and light spectrum to grow what we want them to grow for our needs. Not too much water just enough. Not too much fertilizer, just enough.
Plants need acclimating even when being moved from outside to the indoors. What else were you thinking of growing? Don't do carrots or root crops in 10 cm. Vines? Vine tomatoes or rather indeterminate tomatoes would be a vertical crop?
Peppers do best planted as they are showing. I only know, sadly, one language, so I am going by the pictures. Seeds are planted in a minute amount of soil, too much soil holds too much moisture and causes rot.
It looks like 'starter trays' that have sectioned pots of soil 2" deep and an inch or so wide, holes in the bottom of each. Is this correct?
The rule is plant a seed only 3X the widest dimension on that seed. Pepper seeds would be about 1/4 inch deep. Pea seeds about 3/4 inch deep. Soil firmed under as well as over the seed for proper soil/seed contact.
I have learned to use a spray bottle of water to keep just the top half inch of soil moist, not the entire 2". Too much moisture will rot roots, rot the seeds.
When you see the roots coming out of the bottom holes it is time to up pot into 3" pots of potting soil. This is the time to fertilize lightly, 5-5-5, even numbers.
What is your light source? You have to have artificial lighting or a greenhouse or a protected patio for starts. Where are you planning to grow these peppers?
Is there a way to get this translated? How embarrassing. Sorry!