Measure and mark the position of your rails on 1 of your posts. For a big fence you need a top, middle and bottom rail. Place a picket on top of the plinth and mark on it where the top and bottom of each rail will be. Use the picket to transfer these positions to the other posts. Use your square to run lines around the posts. Mark the depth of the rail on each post. Put a cross on the area of timber to be cut out.
Take measurements between the first 2 posts. If the length of the fence is longer than the timber, make a join in the centre of a post. Make the cut for the corner post at a 45-degree angle. This means the 2 rails meeting at the corner will fit together. Place the rails in the slots and nail them off. Continue along the fence for your middle, top and bottom rails.