Find a flat space on solid ground that can support the barn. Draw a rough plan of what you want your pole barn to look like. Designing your barn in 8-foot sections is best, because lumber is easy to purchase at that length.
Carefully measure to determine where the four corners of your barn will go and make sure they line up, placing stakes at each corner to mark them. Using a shovel or digging equipment, dig a hole at least 24 inches deep and place your post in it. When you have checked the alignment of the post, brace it, mix a batch of concrete and fill the hole with it. Repeat at each corner and for any intermediate posts, and allow the concrete to dry for at least one full day.
Nail 2 x 6 stringers between the tops of the posts, making sure they are fully secure. Begin installing the roof trusses on the stringers, having someone help you if necessary. The first roof truss can be supported by a pole or held up with rope, and additional trusses are braced against the first one.
Attach the boards or plywood pieces for the walls, if your barn plan includes walls. Brace the walls with 2 x 4s.
Nail 2 x 4 pieces of lumber to the outside edge of the stringers, which will help hold up the weight of the roof trusses. Use roofing nails to attach the tin roofing sections, overlapping each section a few inches.