The standard is to clear a circumference to the drip line around your tree. The drip line is the furthest distance of your branches from the trunk. Remove all grass, weeds, etc. You don't want anything in that area competing for food. The feeder roots take up nutrients in this area. You don't have to dig very deep. Just enough to scrape the grass away to expose the bare soil. Place mulch densely in that circumference in the form of wood chips, leaves, grass clippings, straw, etc to keep grass from regrowing in that area. I would pile the mulch at least four (4) inches high to help smother out grass and make it easier to pull up any stubborn regrowth. Put your fertilizer in that area and cover it with your chosen mulch. Every time you fertilize, rake the mulch away, apply fertilizer, water in and rake it back over. I would suggest creating a rim around the drip line to help keep water from running off. If done right, it actually adds to the attractiveness of the tree/lawn.