Scale the building down to size. Choose the largest size your budget accommodates, as it will create a better effect. For example, a six-foot-tall skyscraper can be made to appear far more realistic than a 10-inch tall model. If the models are too small, they will look like models no matter what additional steps you take.
Design the veneer. To design the veneer, you also need to design your shot. So, for example, if your camera will film two sides of the building, you only need to design those two sides. The others can be omitted or made of an un-detailed, neutral-colored balsa wood sheet simply to support the finished veneers until detonation. You can also design access holes in unfilled sides, allowing you to load dust and debris inside the building to be released upon detonation.
Build a scale model of the building you want to explode. Building techniques are similar to architectural models, with balsa wood, glue and paint, cutting pieces with razor-knifes and finish saws. There are additional considerations. Whereas the architecture model is concerned with the proportion and veneer, the movie model, to be exploded, is concerned with intentionally weak areas that will give way first. It is also important to use some building materials that are denser than others. For example, you might to use a balsa wood veneer, but hardwood members inside of the heavier and denser materials will behave differently than balsa wood, creating more realistic flight characteristics of the explosion debris.
Paint the visible facades, adding weathering details. Even new building may have streaks of oxidation, clouded windows or signs of being exposed to the environment. Look through photographs of buildings for reference. Paint interior panels a variety of neutral colors as if they where dust-covered.
Place dust and debris inside the structure so that the detonation creates a dense burst of dust along with flying pieces that mimic the structural components that would be sent airborne in a real explosion. Make the debris general in shape, so it is not recognizable to the audience. For dust, you can use a combination of things, including dust from a vacuum cleaner or soot from a fire -- preferably soot that has grayed a little with water and then dried.
Place small, pre-wired explosive caps in strategic places inside the building. The wires should be completely insulated. Their ends need to be capped. You need to wear anti-static clothing while handling them to avoid a potential static discharge. Place them where there would be structural joints or where the structure would be most likely to fail. You may need to consult an engineer for placement.
Wire the blasting caps from a safe distance to a central ignition control switch. You will need to purchase blasting caps and ignition devices from specialty demolition ordinance suppliers. You will also need significant training in explosive ordinance and an explosives license, or you will need to hire an expert explosives contractor to set the charges and oversee the safety of the detonation and filming.
Enclose your high-speed cameras behind safety glass and secure them so they cannot be knocked over and damaged. It's best to use multiple cameras since you only have one take -- unless you want to rebuild the model.
Clear all personnel from the area. Make sure each person within the area is accounted for and away from your detonation zone. Roll your high-speed cameras and detonate the model.