Step 1: Prepare the ground

Dig over the ground thoroughly and remove all traces of perennial weeds and large stones.
Incorporate extra grit if your soil is poorly drained or if your plants require it - this is particularly important if you're planting bulbs and alpines.
Add organic matter to improve the soil. High levels of organic matter are required by some plants but all will benefit.
Level off the site and break down large lumps of soil.
If you have not prepared the whole area dig a hole for the plant twice as wide and as deep as the original container and improve the soil as above. If the soil is low in nutrients - for example if your soil is sandy - add a light dressing of fertiliser in the planting hole and mix well. Add a further light top dressing after planting but not close to the stem. Do not use large amounts of fertiliser as this can damage plant roots.