A white screen, convex lens on a lens holder and ruler are arranged such that rays of light from a distance object are incident on a lens that is close to the white screen. See the diagram below
The lens position is adjusted to and fro until a sharp image of a distance object is obtained on the screen.
Distance object means an object that is at large distance relative to the focal length of the lens. For instance an object 30 metres from a lens whose focal length is 21cm. The object position is many times longer compared to the focal length of the lens.
The distance between the lens and the screen where the sharp image of a distance object is formed is the focal length and the area occupied by the image is the focal plane of the lens.
Please note that the estimated focal length is not exact but can be 2 cm plus or minus the real focal length of the lens.
This method of estimating focal length depends on the fact that parallel rays from infinity converges at the focal point on the screen.