Ideally water the soil around the plant well the day before you dig it up. Roots generally grow out no further than in line with the leaf canopy above. Use a spade to dig a circle around the tree following the canopy line above. Then lift the plant keeping as much of the rootball intact as you can. You may have to use loppers or secateurs to cut through stubborn thicker roots.
Make sure you cover all the roots as soon as the tree is out of the ground. Large thick plastic sack or hessian are good choices because it’s really important to stop roots drying out. When you move the plant to its new planting hole, check its roots can be spread out fully, otherwise you’ll have to widen the hole further.
It’s also important to plant to the same depth as before (use the old soil mark at the base of the stem as an indicator). This is because planting too deeply will often kill the plant. There’s no need to add compost to the planting hole unless the soil is sandy or very poor.
Then, firm around and on top of the roots as you fill in the soil to the planting hole. You may need to stake the plant if it’s large or being planted in a windy area. Another essential thing to do is to water the plant in well and regularly after transplanting.
Adding a thick layer of mulch such as garden compost to moist soil around the plant will help keep moisture in. It’ll also keep weeds away that might otherwise compete with the plant for water.