I first did a rough crop of the image. If you're careful you can crop it to size and not have to resize it at all. Watch the numbers in the title bar change as you tweak the selection that you will crop. You can drag and fine tune the sides of the selection after you've made the initial selection.

To do a crop you first left click and hold down the button to drag a selection. Click and drag the sides of the selection to fine tune it to size. That selection will be the size of your new image.

Go to Edit, Crop Selection. That will remove everything you have not selected.