Step 1: Taking the shot
APPLE/COMMAND + SHIFT + 3
The keys are highlighted below.
Hold press and hold in that order. You should hear a snapshot sound. I will be taking a screenshot of Pawngame chat.
Step 2: Cropping
APPLE/COMMAND + K
Double click to open the screenshot. Click and drag. The stuff inside the box will be saved, the rest will be discarded.
Now File---> Save