Hi fergie, it would be useful to know where abouts you are in the world and which of the hundreds of magnolias you've bought ??? but here are a few general guidelines. The MOST important thing when planting magnolias is to know that the roots are very fleshy and fairly fragile so NO firming in with your boot as is the norm when planting most plants. Prepare the area where you are going to plant by digging in plenty of organic matter (if your in the UK, check out your local 'greenwaste' company, its a great source of organic, sterile, soil conditioning material) Dig the hole and fork in a handful of fish blood and bone or pelleted chicken manure. If the ground is dry, fill the hole with water and let it soak away. Place the plant at the same level as the compost in the pot, refill around the plant GENTLY firming in and depending on the size, add a stake if required, placing the stake on the downwind side of the prevailing wind (to prevent it rubbing the plant in any wind) and water again. Depending n the subsequent weather, make sure that the plant stays moist until well established.
regards, Lannerman.