Don't be conservative with the mulch and make the area too small by cutting the boundaries too close to the tree trunks. Incorporate at least half of the drip-line area on large trees and all of this space on smaller trees. Try not to create boundaries that project too abruptly, as they will not appear natural and create hard to maintain areas. Before spreading the mulch, get rid of all grass and perennial weeds. Identify the weeds and eradicate physically or chemically.
Several herbicides are effective for home use for most weeds or grasses.
Be sure to observe label directions and avoid drifts by applying at low pressure. Now, decide on the type of mulch to use. The most influencing factor is existing trees. Other recommended organic matter would be compost, leaves, grass clippings or bark. Again, the good part about these natural areas is that when the needles and leaves fall it adds to the mulch and compliments the area.