Instead of buying stock sizes, I rip all the parts from a sheet of 3/4-inch PVC using a track saw
First I pre-assemble three sides of the sleeve on my workbench
The base comes first: I rip the widths I need and cut them to length. As before, I glue and screw three sides together and screw on the fourth side temporarily. I bevel the top of the base to shed water, so while it's still all together, I set the bevel angle on my table saw and run the piece through on all sides.
The top collar is thicker and wider than the other components, so I stack two layers of 3/4-inch PVC glued and screwed face to face. Because this band is purely decorative, I glue and screw three sides together with simple butt joints.
Sleeving the posts. Now I can bring all of the parts for the wrap onto the porch. I start by slipping the pre-assembled part of the sleeve around the post.
I slide the sleeve up as high as it can go and drive screws on all sides at the bottom, securing it with a space between the sleeve and the post. Next, I plumb the sleeve in both directions and drive screws near the top to anchor it. At this point, I can use the screws to micro-adjust the sleeve for plumb.
I install the bases by slipping the pre-assembled part of the base around the post and then screwing and gluing the final side in place. Once the base is complete, I just slide it down to the porch floor and screw it to the sleeve on all four sides.
The top collar is the only component that can be installed in one piece. I swing the free side out of the way as it slips around the sleeve. Now it's just a matter of swinging that side back into place to complete the assembly.
The supporting band is last and is installed much like the base; I slide it up along with the collar and attach it to the top of the sleeve.
Depending on my schedule for installing the wraps, I can plug the holes on the bench or after the wrap is installed. The PVC plugs are made from leftover scraps. I cut them with a 3/8-inch Fuller plug cutter chucked into my drill.
I dip each plug in PVC glue before tapping it into the hole.
When all of the plugs are in, I trim them flush, using a thin, flat saw. I sand the faces to smooth over the plugs and to remove any remaining saw marks. Finally, I caulk all the joints using 100% acrylic latex caulk. The wraps are now ready for paint.