Start from selecting color you like and then select rectangle shape. There are four grippers in each side of oval to resize and reshape it

You can move selection using mouse - simply click and hold your mouse inside the selection area and drag it to the desired location. Don't worry about perfect shapes: focus on selecting the specific part of the image you want, and refine the selection later if needed..

Use to show and hide grid.
Right-click (or long-press) on the gripper to access a menu: "Snap to grid" menu item aligns the active gripper to the nearest grid point, "Make circle" menu item creates a perfect rectangle while keeping the active gripper in place.

When you are satisfied with selection use to open new image-editing window and copy selection to it. In the upper-left corner of this new window find the save button to save your work.