Place your posts correctly and then we'll go onto pergola footings.
Make a frame using battens with small crosspieces at each corner to make the frame rigid.
I've taken a few pergola photos to help me explain the procedure.
Use preservative and treat both ends of the posts. Standing them upright in a bowel of preservative for an hour or so.
Read the instructions on the tin carefully.
While you are waiting, dig the pergola footings for each of the posts.
A hired auger is your best bet. Dig to a depth of one foot for each post.
You may need them longer. Oversize them for now. They will need exact sawing later.
Place the first post in the hole and position it vertically using timber
supports. Repeat for each of the posts and make sure they line up with
each other and are not twisted.
Make sure the posts remain perfectly vertical by using a level. Smooth the concrete down so that any rain will run down away from the posts.
All you can do now is wait for the cement to dry overnight before going onto the next stage.
Once the concrete is dry the next step is to cut the posts to the right size.
Using the lowest post - use a level to mark the top of each of the other three posts. Square round each post using a pencil and saw each post using the pencil lines as a guide.