Motivation I recently bought a microwave steamer and it didn't come with instructions.
Looking for how to use it I didn't find many sources on that topic.
Finally I came up with how to use it.
Thus, I wrote this article to help others.
How it works The process is simple: the microwave heats the water in the lower tray.
The steam that rises from the boiling, which is hotter than the water, will cook the food in the upper tray.
The microwave will also heat the water within the food.
This steam will expand and generate some pressure, that's why some steamers are open to let some steam leak out, other's have a vent or switch you can open or close.
Step by Step

• Put some water in the lower tray.
  Leave some room at the top so that when the water boils it doesn't spill or touch the food.
  
• Arrange your ingredients on the middle tray, the one with holes.
  Space them evenly.
  If you have many ingredients, you can clump them together, but some may become meshed with the others, it depends on the food.
  
• Close the steamer and open the upper vent, if it has one.
  

Tips

• Add preheated water if you want a faster cooking.
  
• You will want all the pieces you are cooking to be roughly the same size so that they cook in the same time.
  
• As a rule of thumb softer things cook faster than hard ones.
  You could have a small potato chunk and a medium fish chunk and they'll cook at about the same time.
  
• If you have two trays, place on the lower one the pieces that take more to cook, like potatoes.
  On the upper tray place the greens, fish and things that cook faster.