Let's cut the long story short. If you know how to sew it's really easy to make yourself some nice and comfortable maternity trousers. I'll try to demonstrate my version in some quick steps.
1. I take the jeans that I like and that I can still put on with the zipper closed but leaving the big button open. I carefully rip the waist/ belt off (carefully - after giving birth I actually changed back the jeans and sewed the belt back on).
2. I cut the front and back pieces for the belly out of nice, soft, stretchy jersey. Here is the pattern taken from a magazine La Mia Boutique - Febbraio 2009 (Italian). I just hope I won't get sued because of that. (I once made my own pattern but lost it in a huge pile of patterns that I have.) (Before I cut anything out of fabric I put the paper pieces on my jeans and I imagine how the whole thing will look like. You can make the paper pieces bigger or smaller according to your jeans size. So don't rush, take your time, measure everything, use pins and pin everything in place. I always do that before I make any final cuts or stitches and it pays off.)
4. I sew the two jersey pieces together on the side seams. !!! But on one side I leave 2.5 cm of open space on two places: right below the fold (on top but on the inner side) and close to the bottom seam again on the inner side. That's leaving space for inserting the elastic band. I mean the elastic band can be optional but it's better to leave room to use it. In my opinion it's good to have it because otherwise I have a feeling that jeans are slowly moving down my body especially when I squat or bend - jersey is just not enough tight.
6. Here I actually inserted the elastic band only on the top part, not yet at the bottom. For now it's OK. I sewed with double needle before I could put in the elastic band, of course. So that's really the final thing. You insert the elastic band and sew it together.