Installing aircraft or any other addons into the Steam Edition of Microsoft Flight Simulator X is pretty much the same process as installing aircraft into the standalone Microsoft Flight Simulator X
Always make sure to read the included readme file if any as it may contain specific or important information about the aircraft or addon.
If the aircraft comes with an installer then you just have to point it to the right path. The path to the steam edition for FSX is usually the following.
Steam Install Path: C:\Program Files (x86)\Steam\steamapps\common\FSX
Steam Install Path: C:\Program Files\Steam\steamapps\common\FSX
Manually installing aircraft files is just a matter of copying the contents of the downloaded archive to the correct folders.
Most of the important files which are contained in a folder usually named after the aircraft will go under the "path_to_fsx\SimObjects\Airplanes" folder. This is usually the folder containing the Model, Panel, Sound and Texture subfolders.
Steam Install Path: C:\Program Files (x86)\Steam\steamapps\common\FSX\SimObjects\Airplanes
Steam Install Path: C:\Program Files\Steam\steamapps\common\FSX\SimObjects\Airplanes
If the downloaded aircraft contains custom gauges then these will go under the following folder paths. Only copy the gauges and not the folder itself to this directory.
Steam Install Path: C:\Program Files (x86)\Steam\steamapps\common\FSX\Gauges
Steam Install Path: C:\Program Files\Steam\steamapps\common\FSX\Gauges