Rhododendrons have a very shallow root system. This is what makes rhodos so easy to shift and is responsible for the statement "rhodos love a ride in a wheelbarrow." If their root system was larger it would be hard to shift a 10-year-old plant. But with their small root system, it makes moving them easy.
Planting into good soil
If you have free draining soil that is rich and good then you can plant straight into it without adding or doing anything special to it.
If your soil is poor
Simply dig a hole 2 - 3 times larger than you actually need, and fill that hole with good soil, peat or rhododendron potting mix. It is important to mix your introduced soil conditioner in with the existing soil, as this encourages the roots to venture out past the hole which was dug. It is a bit like making a cake. I put the soil from the hole into my wheelbarrow and mix the soil conditioner in with that, then use this mix to back fill into the hole.
If you have clay soil
You can plant your rhododendrons on top of the ground and mound up heavy good soil and mulch around it. One of the largest rhododendron gardens we have was entirely on a clay pan. The rhododendrons were all in raised garden beds, none of them were planted into the clay. You can mix gypsum or clay breaker into the existing soil, this will assist in breaking down the clays structure to encourage earthworms in.
Pea straw helps to keep water in the ground and helps to reduce water evaporation by the sun. It also helps to control weeds. We put our pea straw on thick and usually replace every 2 years.
Rhododendrons must have plenty of water. Without enough water in summer rhododendrons shut down their flower production for the following spring. Rhododendrons don't like their feet to be sitting in water, so ensure that the soil is free draining.