There are various types of kettles but they all have the basic same principle, a base and a water container.
Step 1 - Make a mix of water and citric acid (2 tablespoons of warm water with one tablespoon of citric acid). Using a clean toothbrush clean along the top rim and lid if applicable. Rub in small circular motions to remove the tough bits.
Step 2 - Half fill the kettle with water and boil. Let the water sit to cool for five minutes. Once the water has cooled slightly add 2 or 3 tablespoons of citric acid. Leave for a further 15 minutes.
Step 3 - If needed repeat step 2, adding slightly more citric acid and maybe add a splash of white vinegar.
Step 4 - Whilst the citric acid is left to do its magic its time to concentrate on cleaning the base.
Step 5 - Once the water has cooled and the limescale is removed from the bottom of the kettle, use the toothbrush to clean the sides, inside the kettle.
Step 6 - Clean the outside. If the kettle is greasy use a degreaser and rinse well before applying some citric acid mix with the toothbrush. Also it is a great time to change the filter. If this isn't possible scrub with the citric acid mix and rinse well.
Step 7 - Rinse the outside and dry. Rinse out the kettle a few times with hot tap water.
Step 8 - Refill and boil the kettle at least twice, leaving to cool slightly between each rinse, so you don't burn yourself with the steam. Whilst doing this clean the kettle base. Unplug from the wall before you start. Remove grease first with some degreaser and then remove limescale deposits with the citric acid mix. Also wipe cable and plug.
Step 9 - Buff the outside and replace onto stand. Leave the cable flush to the wall or hidden. Always leave neatly and leave a note just in case the kettle needs a few more boils.
Step 10 - Sit back and enjoy a nice brew