Mulching the trees in your landscape with an organic mulch is a great way to improve the soil structure around them, allowing water and nutrients to easily reach there root system.
Before you mulch there is a little preparation that needs to be done. Small to mid size trees should be mulch to about the drip line of the tree, this is how far out there root system travels. Mark out the area that you are going to be applying the mulch. This is best done by taking a tape measure and marking the distance around the tree with something visible on the grass, like garden lime.
Next you need to take a shovel and cut the grass along the marking and completely remove any sod and weeds. When removing the sod, you only need to scrap about 1 1/2 to 2 inches of the surface off, don't dig down. Take the shovel and slide it under the grass. If the area is hard, you can soak it the night before to loosen the soil and make it easier to remove.
Once you have the area prepared, you can apply a 3 to 4 inch layer of compost or a organic mulch of your choice, this tree here is mulched with compost.