Creating a new Template Variable
Once Image+ is installed, you can create Template Variables [TV] in the normal way, but instead of setting the Input Type in Input Options to Image, select Image+ instead.
For most use cases, you should also set the Output Type in Output Options to Image+. This allows you to control the image output on a resource.
The cropping tool
When using an Image+ TV, you could select an image exactly the same way as in a normal Image TV. Once an image is selected, a cropping window is displayed and you could select the area of the image to use. The original image is not changed and so you can upload a single image, and re-use it in several places at different sizes (and crops with multiple Image+ TVs) around the site.
If you want to change the cropping area, you have to click on the crop trigger of the Image+ TV.
In the cropping window you have to drag the handles of the dotted box to change the cropping area. If the TV was configured with size constraints or an aspect ratio, the crop area will be restricted to match this aspect ratio.