Regions
Here's a quick tutorial on resizing regions. Creating new regions is similar.
https://www.youtube.com/watch?v=-q8HcNrbPhA
Firstly you need the tool:
/ng tool
The tool has 3 modes.
It can be used to check area for regions. You just right click any block (from far if you wish) and it tells you if there's a region, whose it is and highlights its corners
You can switch to select mode by clicking left mouse button while sneaking. It allows you to select rectangular area by defining opposite corners with mouse buttons. The color of the rectangle and messages in the chat will tell you more about your selection.
To resize a region you need to stand inside it while switching from check mode to select mode. The corners of the region will turn gold (by default). Next step is to right click a corner. Then you can move it wherever you wish by clicking right mouse button again. You can switch to the other corner with left mouse click and move it as well.
When you have you area selected with the tool, whether you create a new region or resize an existing one you need to execute a command to confirm the action.
/g rg buy
Remember that a valid selection is indicated by red wool (by default)
/g rg list displays a list of regions
/g rg delete <index> removes a region by the index