one thing my husband likes on his dress shirts are channels for collar stays. it's pretty easy to figure out by looking at a dress shirt that already has this feature, but here you go anyways!
[sorry for the following pictures to be of such poor quality. not enough sunlight, having a difficult to photograph fabric, and this ungodly orange table that i sew on are all on my list of excuses. actually, i really love the orange table. in fact i painted it that color long before i ever knew it might serve as a background for photographs.]
first cut two under collar pieces. if you don't have enough fabric to cut the second one in one piece, just cut out two halves; you really only need the outer thirds of this piece. take one of your under collar pieces and mark a line from the collar point angling up the direction you want the channel. then mark 1/4" on each side to give you a 1/2" wide channel. fold down the corner of the collar that attaches to the stand. as you do all this, make sure you check where the seam allowances will fall so that you keep the entire channel opening free from the collar stand once it is all sewn together. once you are confident you have this all worked out, trim away all but 1/2" of the folded under bit.
now you need to fill in the gap that you just folded over, so take the second under collar piece and position it under the first so that it fills in the gap. if you are not using an entire under collar and had to piece it, check the position against your pattern piece to make sure it all lines up correctly.
pin it all in place then edge stitch turning when you get to the stay channel. do this on both sides of the channel. you can trim away what you don't need of the back layer so it only covers the channel, or you can leave the entire piece attached. if you do this, i would suggest trimming off the seam allowance to reduce bulk. depending on what you like to use for stabilizing the collar this under piece could take the place or add to whatever stabilization method you prefer. i didn't trim much at this point, but later i did trim some bulk out of the point. just something to watch for. after all that is done proceed as normal to construct your collar.
i also wanted to show you what a difference it made in my collar construction by cutting the under collar smaller and stretching it as i sewed. the collar naturally curves and ultimately gives you a smoother line. had i trimmed as much width from the under collar as i was supposed to, the curve would be even more pronounced.
i was impressed anyways.