I want to plant a small size sinningia in my office. I read about the plant and found that it requires high air humidity.
How can I maintain a high air humidity in my office with a simple setup?
Really simple is a terrarium. A small aquarium, 5 gallon or less, with a layer of drainage at the bottom (stones for example), a soil separator (landscape fabric or new ladies nylons), and a tropical potting mix.
The key to success is a glass top for the terrarium which retains the humidity in the air. In fact with bright indirect sunlight your problem is more likely to be that fungal agents will ruin the flowers. Simply take the top off to lower the humidity.
I don't know about where you are (pretty sure its hot though), or if your work will let you use one, but a cool mist humidifier is not very expensive and would work for your purpose. If you live in a colder clime you can get a normal humidifier that produces warm humidity.
Many brands of humidifiers are available and are often sold as health products to help people with colds breath better. Get one and keep it running all the time. Note that if mold is an issue for you or your coworkers or is common in your work place adding humidity may be a really bad idea.