Human eye has many transparent parts that acts as a convex lens of adjustable focal length. The focal length can be changed little bit while seeing objects at different distances. For clear viewing, the image should form on the retina. Most common defects of vision, myopia and hypermetropia, occurs because the distance of the retina from the eye lens is less than or greater than what is required. Spectacles make the incoming light beam more divergent or less divergent to help the image to form on retina.
You need a convex lens, a torch bulb/candle, set of convex and concave lenses of different powers.
Fix up a convex lens of large power (say 20 D) in a stand to represent the eye lens.
Keep the bulb/candle at some distance (say 20 cm) from the lens.
Keep the screen on the other side of the lens to get sharp image of the bulb/candle. The screen represents retina.
Now displace the screen slightly away from the lens. This represents a myopic eye where the eye ball is elongated or due to some other reasons the image forms before the retina. You see the image gets blurred.
Put an additional lens from the given stock in front of the eye lens and see the image. This is your spectacles glass to correct the myopia. By trial and error, choose the best power of the lens so that the image becomes sharp. Don't displace the retinal or eye lens in this process.
Now remove the additional lens, focus the image on retina by adjusting its position. Now shift the retina little bit towards the eye lens. This represents presbyopic eye where due to contraction of eyeball or otherwise, the image forms behind the retina.
Put proper power lens in front of the eye lens so that the image forms on the retina.
In myopia, the retina is farther away from the eyeball and the image forms in front of the retina. The beam is to be made little more divergent so that the image shifts away and forms on the retina. Thus concave lens of appropriate power is needed. Similarly, convex lens of appropriate power is needed for presbyopic eye.