I like to start with the plunger because it is the key to the Water Hand Cannon. If you can make this part the remaining parts will be easy.
Start by taking the 1/2" PVC end cap and press fitting it on the 1/2" PVC scrap. Make sure the cap is properly seated and the pipe is pressed as far in as possible. The tapered cap should provide enough friction on the pipe that it will not slip when being turned down in the lathe. Now using the 1" PVC scrap as a guide remove material until the cap fits loosely inside. Make sure to remove enough material so that the cap will not bind. I have about a 1/6th to 1/8th inch gap. Now take a file and remove any burs leftover from the turning process. This is also a good time to remove any writing that may have been cast into the face of the cap.
Cut a section of 3/4" PVC about 4 inches long. Now using the 1" PVC guide remove material until the 3/4" pipe fits into the 1" pipe. Cut the turned down section of 3/4" pipe into one inch sections. Then using a boring bar open the inside diameter so the 1/2" PVC scrap will fit snugly inside. You only need one of the one inch sections, I called for a 4 inch section because it was easier to hold in my lathe.
You can also buy 1/2" PVC connectors but these will have to be turned down just like the cap was.
Cut a 2" section of 1/2" pipe. Using the round nosed bit place two groves in the center of the 2" section. Place the grooves about an 1/4" apart. When cutting the grooves always remove a few thousandths of material and then stop the machine and place an o-ring in the grove. Check the fit with the 1" PVC guide. You want the o-rings to have a tight fit into the 1" PVC. The tighter the fit the less leaking but too tight an the rings will bind.
The difficult pieces of the Water Hand Cannon have now been constructed.