We have a 4-burner gas cooktop. Some of the burners work better than others; the left-front burner is usually easiest to use
To light a burner press the knob down and turn it all the way to the end stop. This will enable full gas flow and activate the ignitor, which you can hear rapidly click. After a second or two the flame should catch and light the burner, after which you can set the gas level as desired
When done turn the knob all the way back off
When using the front-left burner the exhaust fan will automatically activate. To activate it manually use the toggle switch near the gas knobs