How to mark a horizontal line on a wall without a level?
Take two stools, place a board, a piece of plywood, or whatever else you have on them. Take a deep bowl, at a distance of about 1-2 cm from the edge, draw a line from the inside at the same distance from the top with any indelible felt-tip pen, pencil, or scratch if the bowl is plastic. Pour water into a bowl flush with your line and place on top of the board. You have reached Elementary Coarse Level. If the board is horizontal, the water in the bowl will be flush with your line, if not even water, place anything under the board until the water levels out. At the level of the leveled board and draw a line, then measure anything from it in any direction. You can take a thin hose from the dropper, put the remains of a transparent fountain pen on it from both ends (only the same on both sides), put a risk on these handles at the same distance from the ends, pour tinted water inside and ensure that the water in the tubes is at the same level , by risk. This is also a level, they can transfer the same mark from wall to wall, but with a plate it is easier.
As for "no level", then the option from the transparent hose described in the previous answer is fine. As for "outline the line", we did this: when you already have points between which this horizontal line is needed, we took an ordinary twine and painted it in dry blue powder, then fixed the ends of the twine between the points between which we needed straight line. Then they pulled this string from the wall like a bowstring and let go. After hitting the thread against the wall, a perfectly straight line of blue color is obtained, even on a not very flat wall.