I've traced my pants blocks onto cardboard, I have one set with darts and one set with no darts. The different seam lines are just for woven and stretch fabrics. When use a stretch fabric I drop the rear crotch and take in the sides. You don't need to put it onto cardboard but I find it easier to see under my pattern.
First step (This is the back of the pants). Mark the stitching lines on the top of the pants piece and the bottom of the waistband then align.
I slide my pants block under at this stage and make sure the grain lines are parallel.
I usually line up the crotch line on the pants block and the pattern, this way the inner leg length stays the same length and doesn't need adjusting later.
Adjust the pattern pieces to match the pants block (you can see my lines in red texta)
I extend my lines downwards following the same shape as the pattern to add extra width because I usually need it.
Same thing with the front - Line up waistband and pants stitching lines.
Make sure the grainlines are parallel
I usually have to make a fold in the front panel to line up the front and drop the centre front waistband height.
After I've adjusted the front and back I walk the seamlines to check that it all matches (I used to skip that until I made 2 pairs of pants that fit kind of weirdly and I realised it was because of the seamlines not matching up properly).
This was an easy pattern to adjust, usually I have to drop a lot more in the front and raise the centre back a lot more. If a pattern has pockets I just overlap them and line up the stitching lines like we did with the waistband and top of the pants.
I should mention too that even with adjusting the flat pattern I still cut out the fabric with large outer seam allowances (about 1" extra) as a fitting allowance, once its basted I try it on and adjust to fit. Its always easier to take it in than to try and add width!
I can do more of these posts with different patterns and style of pants if anyone is interested?