To try and visualise the formation of a black hole you will need a sheet of cling-film (US: plastic wrap), a tennis ball, an orange or plum and a lead ball bearing or marble (or similarly weighted objects that you can find around the house), a friend to help you, and a good imagination!
In this experiment the cling-film represents the dimensions of space along one edge and time along the other, and the spherical objects represent a star of different masses as it goes through the stages of collapse into a black hole.
Hold the piece of cling-film flat in the air by the edges and make sure that it is pulled tight. Place the largest but lightest ball (e.g. tennis ball) on this piece of cling-film. It should begin to sag in the middle and the amount of sagging gives the sheet a third dimension which represents gravity.
Now imagine the ball undergoes a supernova explosion and the remaining mass forms an object smaller in size but much denser. Replace the tennis ball with the lead ball bearing (or a sphere which is much denser or heavier than the tennis ball). This should make the distortion of the space-time sheet even greater, representing an increase in gravity at the surface of the ball. If you were to continue to reduce the size of the ball, but keep the mass constant, that is, using more and more denser objects, eventually the sagging would cause the cling-film to break. In real life, the collapse of the star and its increase in gravity would deform the local space-time area so much that light can no longer escape, creating a black hole.
Unfortunately you won’t be able to make a real black hole at home. To do that you would require an object the mass of the Earth – about 6,000,000,000,000,000,000,000,000 kilograms – crushed into a sphere a few millimetres in diameter!
From left to right these images show that as progressively smaller but heavier objects are placed on the cling-film the sagging in the space-time dimension becomes greater, representing an increase in gravity. When a star undergoes a supernova explosion, its gravity increases so much that space and time becomes distorted and a black hole is formed.