Thanks for the quick answer.
So, I put the plate horizontaly, mark one end with a pencil, add a drop of the mixture in the center of the drawn line, dry it so it will be adsorbed on the surface of the plate, I place the plate in a tank filled with water (now vertically), and then I close the tank. Is this correct?
And about this: "When the spot of mixture is dry, the plate is stood in a shallow layer of solvent in a covered beaker. It is important that the solvent level is below the line with the spot on it."
How the capillary action occurs when the beaker with the solvent that is in the tank is covered, and how the vapors escape the covered beaker?