Here is the toilet cistern, the big red plastic vessel is the syphon, this is the part that sucks the water from the cistern and deposits it in the toilet pan, making it flush. Behind this is the inlet valve or ball valve.

To make this easier to see the inlet valve has been removed from the cistern.

The first thing that we need to do is to isolate the water supply to the cistern. This can easily be done by turning the isolating valve to the off position with a flat screwdriver. This valve is now in the off position.

If you do not have an isolation valve or stop top on the pipe feeding the cistern then you will have to isolate the water elsewhere Isolate the water supply 

Once the water is isolated you can flush the toilet to empty the water from the cistern. There will still be a couple of inches of water in the bottom once it has been flushed. The best way to remove this is with a sponge. Push the sponge into the water until it soaks some up and then ring the sponge out into the toilet bowl. You can now unscrew the top nut that connects the water pipe to the water inlet valve, then undo the retaining nut. The valve will now lift clear of the cistern.

Before fitting the new water inlet valve ensure that the rubber washer is in place and that it is facing the correct way, like in this picture.

Push the new inlet valve into the hole and attach the retaining nut.

Tighten the retaining nut with either the correct size spanner or an adjustable spanner. Do not over tighten it! Reconnect the water pipe to the inlet valve using a spanner, ensuring that the valve does not move.

Now that the new ball valve has been fitted we need to adjust the threaded plastic bar, this prevents sidewards movement of the float valve. To adjust it slacken off the nut and then turn the bar so that it touches the side of the cistern and then tighten the nut so that the bar is locked tight against the cistern.

Slowly turn the isolating valve back open and check for leaks, if there are any leaks re tighten the inlet valve retaining nut.