Snap a chalk line across the fascia, dropping it ¼” per foot from one end of the shed to the other. Fasten gutter brackets at 2-foot intervals using sheet metal screws, so the top of each bracket aligns with the chalk line. Slide each gutter section into place, overlapping the sections by about 2 inches. Use short sheet metal screws to hold sections together. Attach the drop outlet at the low end of the gutter.
Place a cap at the end of the gutter to force water through the drop outlet. Attach an elbow to the drop outlet with a short length of downspout to carry the run to the wall. With an elbow, join a longer section to carry the water to the ground. Anchor the downspout to the wall with brackets using plastic anchors in masonry or sheet metal screws in wood.