Half fill the hole with water and tip in a bag of rapid set concrete, watering as you go. The concrete should be filled to the top of the hole. Ensure top of post is at correct height for placement of arch, cutting off level with a chainsaw if needed. This post will become the guide post. Step 4 Install the three other posts one at a time.
Post position needs to be very accurate to accommodate the metal roof.
Set up string lines and check parallel and diagonal distances to ensure measurements are accurate within 5mm. When you are satisfied that each post is in position and vertical, set it in place with rapid set concrete as in step 3. You can then use a water level to mark level on each post, measure and mark the top of the posts and cut off level using a chain saw. Have a helper hold the other end up against the next post and mark where the water finishes. Water naturally finds its own level so the level is the same at both ends.