Right-click on the image and select Tools, then Transform Tools, and then Crop & Resize. Your cursor will change to the Crop cursor, which looks like a knife. You can also select the crop tool from the Toolbox. Dragging the box does not have to be accurate, as you will be able to adjust the box manually. Click the boxes in the corners or on the sides to adjust from that point. To make fine adjustments, use the Tool Options in the bottom of the Toolbox. You can change the position of the box on the image by adjusting the numbers in the Position fields. You can fine tune the size of the box by changing the values in the Size fields. Once the adjustments have been made, crop the image by clicking the center of the box you have made. Everything around the image will be deleted, leaving just what was inside the box. If you are unhappy with the crop, you can undo the action by press Ctrl+Z.

TL;DR in Indonesian:
Bukalah gambar yang akan dipangkas. Tarik kotak ke area gambar yang ingin disimpan. Sesuaikan kotak piksel demi piksel. Pangkaslah gambar.