Turn off the power at the consumer unit or breaker
Verify the circuit is dead with a suitable voltage tester
Remove the existing switch faceplate
Take a photo or note the existing wire connections
Loosen the terminal screws and disconnect the wires
Check that the dimmer switch is compatible with the light type and load
Connect the wires to the dimmer terminals exactly as instructed by the manufacturer
Tighten all terminal screws securely
Fit the dimmer switch back into the wall box
Secure the faceplate to the wall
Restore the power at the breaker
Test the dimmer switch and light operation
If unsure or if the wiring is unusual, use a qualified electrician