How to clean car interior

Remove the floor mats. Vacuum the seats, and the carpet. Using the proper attachment, reach under the seats, around the pedals and the area between front seats and central console. Vacuum floor mats separately.

Use soft brush attachment to vacuum the dashboard and doors. Be careful not to damage knobs vents and sticking parts. Use the same attachment vacuuming the seats. Leather seats are very easy to damage, be careful no to scratch them with the vacuum hose.

There are number of upholstery cleaning agents available. Here I used a spray cleaner from Turtle Wax. Simply spray evenly on the seat or door upholstery and rub vigorously. Then, wipe it thoroughly with a dry cloth.
If you don't have an upholstery cleaner, simple laundry detergent will work as well. Just mix some detergent with warm water, dip a clean cloth into it, wring out good and then just wipe the seat. Work harder on dirty areas. Then, rub dry with a clean, soft cloth.
Don't soak the seat with water, it will cause stains on the seat and mould smell.

Clean the carpet the same way you cleaned the seats and upholstery.
Spray evenly with the carpet cleaner and rub vigorously. Then, wipe it thoroughly with a dry cloth.
The laundry detergent will work on the carpet as well. Damp a rag with a little bit of water and detergent, squeeze the excess of water and rub the carpet vigorously. Then, rub dry with a clean dry cloth.
Don't soak the carpet with water, it will cause mould smell and corrosion under the carpet.

Clean a dashboard, central console, and other plastic parts with a slightly damp cloth. Use very small amount of laundry detergent to remove the stains. Dry with a clean soft dry cloth. To make it shiny, spray plastic polish (I used STP here) and spread it evenly with soft brush. Don't spray directly on the stereo, use brush instead.

Dry-polish gently with clean soft dry cloth. Polish not only makes the dashboard shiny, it protects the plastic. All you need to do after, is just use a soft duster periodically and your dashboard will look like new for a long time.

If you experience that unpleasant musty smell from the vents when the air conditioner is turned on, you can try one of the odor treatment you can buy in your local auto accessories store. It kills the bacteria and removes that mildew smell. Here I used one from Wynn's that you simply spray into outside air intake vent (check directions on the can).
Clogged air conditioner drain tube and accumulation of leaves and other debris under the cowl cover also may cause damp mildew smell. Ask the mechanic to check it when you do your next oil change.