Here's my method, which I like because it seems bulkless to me. It is a combination of several methods, some picked many years ago from the Burda illustrated course, some from sewing tips in a French magazine. I think it was Marie Claire but I'm not sure, this was 20 years ago!
First of all, my apologies because I forgot to take pictures at the beginning, therefore I must replace them with sketches.
1) If your pattern doesn't have a separate piece for the undercollar, make yourself one, by copying the upper collar and trimming 1/8 inches (3mm) all around. Sew the upper collar to the undercollar (my undercollar is cut on bias), stretching the trimmed undercollar to match the upper collar. Stop at the notches (marked with "stop here" on my drawing). Trim seam allowances if needed, turn, understitch and press. If you match the neckline seam on the uppercolar and undercollar, you will see that the uppercollar is bigger. Now fold that collar, still matching the neckline seams and put it around your neck. See now why the uppercollar is bigger? To adjust for the so called turn-of-cloth. The collar folds and rolls beautifully around your neck.
2) Sew the uppercollar to the facing (the collar's neckline seam to the facing neckline seam).
Trim, clip and press that seam open, like me:
3) Sew the undercollar to the jacket. Again, trim, clip, press open:
4) Sew the facings to the jacket.
5) Now, fold your collar and put one seam allowance of the uppercollar and one seam allowance of the undercollar together (the other two seam allowances are inside the collar). Sew those seam allowances together, either by hand or by machine (using a zipper foot in this case). Turn and press. Isn't it beautiful? Light and without bulk.
Peaking inside my WIP jacket... I've used Marcy Tilton's article for Threads, Armani jackets: The inside story:
I taped the lapel roll line with fusible tape, about 1.5 cm shorter than the roll line. The majority of the ease is in the middle third of the lapel line and the easing is done by steaming.
I stabilised the center front edges with 5/8 (1.6 cm) tape which is actually a lining selvedge. Tha tape was sewn with a basting stitch, without stretching. The tape is clipped at roll point, to allow the lapel to roll.
The armhole is stabilised with 1/4 (6 mm) twill tape - the tape is slightly stretched while sewn, thus forcing the armhole to "hug" the body - and at the shoulder, the seam allowance is pressed open for 2 inches (one on both sides of the shoulder point).
I'm also planning to attach a floating chest piece made out of canvas cut on the bias and a wigan strip (also cut on the bias) at the sleeve hem.
Stay put for the completed jacket and more inside pictures.