NPC tool is a Mappet item which allows to simplify NPC spawning and editing.
To acquire an NPC tool, go to Mappet's creative tabs, and grab the NPC tool item.
When you grabbed a brand new tool from the creative tabs, first, you need to configure the tool by right clicking in the air. A menu like this will appear:
There you can pick the NPC's ID you want to spawn and its state.
Once you picked the ID and state, you can close the menu, and can right click on the ground to spawn an NPC by that ID and state. It will spawn an NPC and open immediately individual NPC editor:
This will let you change all of NPCs properties immediately. If you don't want to immediately edit NPC's property then right click on the ground while sneaking (by holding Shift), then an NPC will spawn without opening the individual NPC editor.
You can also always right click on an NPC to edit its properties. Once you finished editing its properties, close the menu, and the NPC will get updated.
If you want to remove an NPC in a humane way (without killing with an axe), right click an NPC while sneaking (by holding Shift).