Dig a hole about 1.5 to 2 times the size of the plant pot. Mix a little compost/leaf mould into the soil you have dug out. Release the plant from its pot and gently tease the roots out a bit. Place the plant so the soil of the plant is level to the top of the hole. Back fill and firm down and water well.

hope this helps.

Hydrangeas can get pretty big Dave. Haven't you got a bigger bed somewhere? You can't keep them small by pruning as you would cut off next years flower buds.