Start digging when the soil is damp, if possible. If there has not been any rain, water the area heavily a couple days before you plan to dig the holes.
Mark out the spots for the posts with the lines for the direction of the vines running from north to south if possible, for the best sun coverage during the day. Keep the vine posts at most 20 feet apart.
Dig holes for the posts 3 or 4 feet deep, depending on the type of soil. Heavy soils will hold the post in place with a 3-foot hole while looser soils might need a 4-foot hole to hold the post steady without any additional anchors. Create a space that will allow the post to sit at a 60 degree angle away from the opposite post.
Set the posts into the hole, holding them straight while another person fills in the soil around the post. Every 6 inches, use a pole tamper to pack the soil down hard around the post until you have filled the hole.