Simply extend the raiser in width and length to suit the leg spacing on the base of your chair. Place the chair on top of the raiser. Adjust the raiser so that it fits closely up to the legs of your chair. Tighten the four clamping wing nuts on each side of the raiser. Test for safety and stability before use.
To Open raisers to the larger measurements, you will need to remove the wing nut and bolt and re-fit them to the second hole in tin arm, making sure the bolt is into the groove of the timber arm.