For want of a better place to document the process, here's how we make our hydroponics runs. We make them in 2 sizes; this is the 150mm x 100mm version, using a 3m length of PVC rectangular downpipe. The ends must be cut off square and smooth so the endcaps fit.
The downpipe is marked off starting 70mm in from one end every 230mm with the seam (if there is one in the downpipe) uppermost. A mark is also made on the underside corresponding to the first 70mm mark. This is for the drainage hole underneath and an inspection hole on the top.
Now, as a surprising amount of hot crap gets flung around by the drill at this point, even I can be persuaded of the benefits of wearing safety glasses and tying my hair back. Check the fit of a pot after drilling the first hole, just to make sure you're not about to wreck a fortune in downpipe.
Once the holes are drilled, take off any wiry edges with a de-burring tool, rasp or whatever so the pots fit in neatly and the gardener won't cut themselves. Then clean all the bits of crap out of the downpipe. A shop compressed air line is handy there!
When the troughs are mounted, I drill a hole at the high end to match the inlet pipe I'm using. This is left until later so the hole goes in the most convenient place :) Then of course we need to plumb them in, but that's another story. Ⓥ