Typically I machine stitch with the right sides of collar and neckline together, and then flip it but I would like to learn the method shown in the video.
My patterns have 3/8" SA all around including the shoulder, neck line, and arm hole seams. I can trim 3/8" in the edges of the under collar and attach it that way but there isn't much of the neckline to sew to.
Would it be correct if I extended the neckline SA of the front and back patterns another 3/8" to give the collar more body to sew on?
Thanks
Yes that is correct, I have not drafted the pattern using the video but these are patterns I already had and wish to use. My patterns do not have any inlay in them so I think it will be necessary to add some extra to the neckline of the front/back patterns.. but I am not sure where and how much to add.
It's just my best guess but I think the principle is the same no matter what system you have used. You don't need to add extra to the pattern itself, all the extra inlay for possible alterations and for attaching the under collar are added when you chalk it out on the cloth. Then there will be enough to secure your undercollar which is sewn round the neck line and secured under the lapel and with the bridal. The undercollar itself is a generic piece that you finish shaping once it's attached and you're happy with the position. So as far as my knowledge extends I think that what is already there is fine and inlay will make up the rest.
If you've followed Rory's drafting method then you don't need to add anything to the pattern. Watch the videos on striking the cloth where inlay is added. When you come to prepare for attaching the under collar you chalk in your sew lines and then use them to align the under collar. The under collar is felled on along the sew line so I think if I've remembered rightly then your 3/8 SA plus the small amount of inlay you add when you strike the pattern onto the cloth will give you everything you need.