Depending on your fabric choice, it is common for one layer of the collar to have fusible, and the other layer to have no fusible. If this is the case for you, the piece with fusible is the underneath layer and the layer without it is the top layer. If you have fused both layers then it doesn't matter which is which.
Place both collar pieces on top of each other, right sides together. At the machine the layer with fusible is underneath, you should be looking at the wrong side of the unfused piece. (because the fused piece will not stretch or move, however the unfused piece may stretch, so it is always easier to handle with the unfused piece on top)
Stitch a 1cm seam around the outside edges of the collar. Ensure to keep the needle down and pivot at the corners.
Now layer the seam allowance - a common method for this is to trim away the seam allowance to half, on one piece only (usually the fused layer). Then clip off the corners on both layers so it's easier to turn right side out.
Turn the collar out to the right side. Have the unfused layer on top and give the collar a press. You want to press the collar so the top layer rolls slightly over the line you have just stitched, that way you don't see the seam.
Some people like to cut their top collar a little bigger than their under collar to achieve this, however unless it is a very heavy fabric this is achievable simply with pressing. Notice in the picture above, the gap that has formed on the unsewn section? That is simply from pressing over the stitch line, dont try to make it sit in line with the other edge.
You can can topstitch your collar now if you choose to (this is purely a style decision. I have topstitched mine with a 5mm stitch)
Lay one collar stand piece down in front of you (right side up) and then lay your collar on top of it. There are notches on the top edge of your collar stand, the edges of the collar should meet them when you place it on top.
Now lay your other collar stand on top, right side down (you should be looking at the fusible on the stand
This creates a collar sandwich. Pin through all layers to hold the pieces together.
Stitch a 1m seam around the curved edge of the collar stand, ensuring your collar is caught between the stands. It's important to back tack the ends of the collar stand as you will need to handle it a lot for the next stages and it has a tendency to unravel.
Layer the seam allowance the same way you did for the collar, but around the curved edge you need to clip small triangles out of the seam allowance (through both layers).
Turn your collar stand out the right way. Now your completed collar and stand should look something like this. Don't be tempted to topstitch the collar stand at this stage.
If you have a loosely woven fabric is is a good idea to stay stitch your neck before you attach the collar, as it can easily stretch out of place and your stand won't fit.
Turn the stitch length on your machine up to the highest length (biggest stitch) possible and do a foot width stitch around the neck edge. Make sure your neck is now measuring correctly, (you can place it on top of your pattern to check they are the same, if you are like me and too lazy to measure)
If the neckline has grown, you can pull the stay stitch thread lightly, almost as you would if you were going to gather it, and that will bring the neck back to measurement.
Lay the shirt out in front of you with the right side up. lay your collar on the neck opening of the shirt, be careful that the right side of the shirt and the under layer of the collar are facing each other. So you are looking at the top layer of the collar. Pin one layer of the collar stand to the neck edge, keeping the other layer free.
Make sure you collar stand lines up exactly with the edge of your button placket.
Stitch the single layer of the collar stand to your neck edge with a 1cm seam. Make sure you stitch right to the ends, and back tack. It should like this from the outside.
This next step is very hard to illustrate, but essentially, you want to wrap the collar stand around itself so its right sides together again. Its slightly awkward to do, and you will need to push part of the collar inside to form a little burrito. Pin the base of both layers of the collar stand and neck together, ensuring nothing extra (like the collar) is caught in that section.
Stitch a 1cm seam along the neckline edge for approximately 4-5cm, or whatever you can manage before it starts to get bunched up under the machine.
Clip the seam allowance on the lower neck edge.
Gently pull the collar stand right side out again. Repeat the process for the opposite side of the collar stand. You now only have an opening in the middle section of the collar stand.
Fold the remaining open edges of the collar stand under by 1cm and pin in place so it just covers your original seam line.
Edgestitch all the way around the collar stand. This will close the gap you have left in the neckline. Its easiest if you edgestitch from the inside of the collar stand. Pick a shoulder seam and work all the way around the collar stand, you will end up finishing where you started. There's no need to back tack, just carry on sewing for 3 or 4 stitches over your first few stitches.