I don't know how to do floor glute-ham, but I know another jimmied version. Take a flat bench, face it horizontally to a Smith press, set the bar on the Smith press to wear there is enough room between the bar and the bench you to put your feet in. Then, secure your ankles under the bar and rest your knee/lower thighs on the flat bench. From this position you can perform some bastard glute-ham raises.