Firstly put a stake at the centre of the pergola site. The number of posts that you need to put up will depend on the size of the circular pergola. Say you want a 10 foot diameter pergola. That one will need 6 posts. From the centre draw 10 foot diameter circle showing the inner diameter of the pergola. Mark the 12, 2, 4, 6, 8 and 10 o’clock positions on the circle. Dig 8 by 8 by 18 inch holes at each of these places with one side along the circumference and the rest outside it. Make the bottom of the holes flat and insert an 18 inch long prefabricated concrete base with a bolt attachment in all holes. Now insert a 6 by 6” post into the concrete base making sure it is completely erect. The top of the posts should be at the same level. If they are not they should be cut to the same level.  On a graph paper draw the circle of top beams for the pergola. You will need to cut out 6 curved pieces as beams for the pergola. Place the beams on the posts of each post and secure them with bolts. For a beam take a 2” by 4” by 12’ boards and attach it to two opposite posts. Cut some 2” by2” boards for rafters. The maximum length will be 10 feet. Then it will decrease. From the graph paper you will be able to have an estimate the number of rafters needed. Finally use the pergola plans you have to construct an appropriate deck for the pergola.