I would either (depending on weight) mount it into a barrel of concrete or, if it's heavy enough, to a large lump of heavy wood ie; an oak stump. If you can, weld 4 right angle lugs to the bottom edge of the steel so you can bolt it to the wood.
Height wise, ideally you need to have the surface at 'knuckle' height. If you stand up straight, arms down by your sides & curl your right hand into a fist, your fist should touch the top surface of the anvil without you having to bend over, or flex your elbow. If you can get this height it will seriously reduce fatigue in your hammer arm, back & shoulders as you wont be stooped over whilst working. It also makes it easier to keep your hammer face level.