Get a carpet cleaner with an attachment to do stairs and other narrow areas. Take the seats out and do it properly. Use laundry detergent capsules. Does the same job as expensive stuff. First go over all the carpets with a good stiff bristle brush to agitate it.
Use hot water. Like very hot, in all stages.
Keep going over the carpet with the nozzle attachment until you see all the water being sucked up has turned clear.
Dry the carpet with a towel and then a hairdryer.
Clean the seats very thoroughly with the carpet cleaner if cloth, or leather cleaner if leather. Treat it to a really good dose of leather conditioner and let it soak in over night.
Regards the headliner. Bit more difficult as it's above you and, you're gonna get wet! Patience and time with the brush and hit water.
Once the carpet is dry give it a good douse with fabric freshener. Don't bother with shake n vac. It's a nightmare to gt out of automotive carpet.
Take everything out of the boot. Wash the wheel well and the surrounding area with mild bleach solution and a face cloth. That you don't use. Clean the boot liner in the same way as the cockpit interior.
Spray dashboard cleaner inside the boot and wheel well and buff until it's not glossy and slippery.
Clean the door cards and doors, dashboard, etc with a good automotive spray cleaner of your choice.
Airfreshner.
Done.