Finish up the interior job by cleaning the air vents with cotton swabs and maybe a little accent spray. Just be careful not to use too much. Clean the inside of the windows with a high quality glass or plastic cleaner. And remember to put the floor mats back in the right place.
Moving on to the outside of the car. This is where first impressions are made. The first thing to do for the exterior of the car is to give it a good wash. Use carwash detergent, not just a household product. Work in sections and pay special attention to lower panels of the car. Rinse with a garden hose minus the nozzle and dry the finish with a soft chamois or cloth. Next take a look at the wheels. Using a brush made for washing wheels is an excellent way to ensure that the job is well done. Looking good so far. Machine buff the finish and proceed to the waxing part. Either waxing or polishing the exterior is what really makes a car shine. Avoid the hard plastic pieces on the exterior. After the waxing is finished open the hood and protect the engine with plastic wrap. Close the hood and coat the whole car in a diluted cleaner. If you like a matte finish hose it off and dry with a clean towel. If the shiny look is what you are after, hose off and let it dry naturally. Finish up with a tire dressing on your wheels. Once again the choice is yours. You can go for a high glossy finish or a matte look.
Following these steps will have your car standing out and looking good. And the next time you see your brother-in-law, you can compare details on how superb a job you did without paying detailer prices.