Well here we go.
First things first.
I always interface both collar pieces and both band pieces. Fabric has life and stretch and grain. If you are going to attach two identical pieces together, things are just going to work out so much better if you have set up the same way.
An un-interfaced collar is apt to stretch and grown a bit more than an interfaced collar. So it is completely inevitable that when you try to attach them, the un-interfaced unit is going to be a little bit bigger than the one with the interfacing.
Why set yourself up for trimming and stressing and trying to ease one piece into another, or even worse, having to unpick little pleats that have appeared in the stitching?
Interface both collars and stands and you will have the security of attaching like things to each other.
Of course you don't want to over stiffen your collar. I did that once years ago and ended up with red marks on my neck.
You get around this by using a lighter weight interfacing because you will be working with two layers.
There is also the issue of keeping the interfacing out of the seam lines. Interfacing in the seams will just fight you the whole way, and no amount of pressing the finished product will make it dissolve.
Here are your seam allowance interfacing options:
1. Carefully trim away exactly 1/2" from around your interfacing so, when attached, 1/8" will be caught in the stitching - best to have it caught and not free floating. I do not do this because it is boring and I am inaccurate and also I am trying to sew stress free with a wandering mind.
2. Use a light weight fusible like a knit or Sewer's Dream, a really lightweight knit I had arrive in the mail yesterday on your recommendations. The idea here is that the interfacing is adding some body but it is flexible. If you interface the whole collar and band you will probably be OK with this providing you trim the seam allowances down.
3. Use a lighter woven interfacing, like I did with this shirt. The woven non fusible adds some crispness and is easy to remove from the seam allowances. As I showed a few posts back attach it, cut the same size as the collar and band pieces, with some quick hand basting. After I am done stitching I trim the excess out of the seam allowances as closely as I can with my scissors.
Now onto the collar and band:
1. Step one is to complete the collar:
Stitch, trim out any woven interfacing from the seam allowances if that's what you are using, turn, and press.
Top stitch around the edges - I use my edgestitching foot so that's a no brainer. When it comes to making sure your stitches don't pack up at the corner, just swing the other collar end under the back of the foot as a "Hump jumper" to keep the foot level - it will be the perfect thickness.
2. Sew the collar bands to the top of the shirt. sandwiching the body of the shirt in between them:
I do this in two stages, one collar on and then the other, because the less fancy you get the easier and more fool proof it is. You can see here where I have trimmed the interfacing from the seam allowances.
3. Next do the ends of the bands:
I just fold the front out of the way a bit and pin it still then stitch each curved end. I back stitch near the bottom but don't back stitch when I stop close to the collar opening. I use small stitches because small stitches go around curves easier and by not back stitching at the end I can unpick more easily if I have to.
I take it easy and like to check at this stage to make sure the stitching starts right at the button band (you should be able to feel it) and restitch until it is right. You want that collar end to flow right into the shirt button bands.
I always stop this stitching a safe distance away from the collar opening. I can catch and close that later with top stitching and I like to have a little wiggle room when it is time to put in the collar. Less stressful this way.
Here is the collar end stitched:
And here it is after trimming just before each end is turned and pressed. At this point I would also press under the seam allowances of the collar opening.
4. Drop in the collar:
Slide the collar into the opening. It is really useful here if the opening is a little bigger than you need. Pin the collar to the inside neck band and stitch it in - it would probably be tricky to get the stitching right to the end but do your best, go as far as you can and don't worry at all about that extra unstitched inch or two, you will catch that later in top stitching.
5. Hand stitch the outer band to the under collar:
Don't freak out it's not a lot of hand stitching and it is under the collar right? The nice thing about hand stitches is that you can take them out easily as you go, which cannot be said of machine stitches.
Here is how that looks pinned in position from the under collar side:
Now when you hand stitch this down the secret is to take a tiny stitch with your needle and to hide the needle under the fabric before you take the next stitch. When you can't see the needle you won't see the thread:
If you don't pull that thread too tight once you press the band the stitches will disappear and look like this:
So that's it, a piece of cake.