Sit straight on the chair, described above, in front of your keyboard. Then rest your hands, where your index fingers touch F & J.
Then move your forearm away from the keyboard to the right, holding your back straight, until your forearm is perpendicular to your chest. This where your mouse (and mouse pad with wrist support) should rest. It varies from person to person, but this ensures your right arm (and shoulder) are fine.
If you are left handed, just do the same, but on the other side. If you are broad shouldered like me, this really helps.