Firstly, you get your collar piece and make a new collar piece like so;
race your collar piece with the centre marked (at right above), then flip over the pattern piece and re-lay the same piece, overlapping at the SEAM ALLOWANCE line (1.5cm in from the cutting edge) If this isn't marked on your pattern piece, you can mark it on yourself for accuracy (I haven't because I'm slack). Then trace around this piece again in its new position, and at the 2nd centre (at top left in the photo above) ADD A NEW SEAM ALLOWANCE, seen at the very top. This whole final thing is your new collar piece. The final centre of this new collar pattern piece is where it was in its original position when you first laid the pattern piece down, on the paper edge at far right in the photo above (I've marked it in the photo but it may be difficult to see my scribblings...) Cut out your new collar piece, placing the centre on the fold.
You will end up with a collar piece that has one portion on the grain (the first one traced, your upper collar) and one portion on the bias (the second one traced, your under collar) Now, with right sides together, sew the seam allowance of your 2nd (under collar) centre, you know, the one you added a seam allowance to when tracing the pattern. In the photo above the unpicker/pointer is pointing to this seam.
Now open out this seam and press, then with right sides together sew the outer collar edge seam. In the photo below the under collar is facing up and my unpicker/pointer thingy is pointing to this new seam.
Now clip and grade your seams as per usual, turn and press, blah blah. In the next photo I've added the collar stand. Not quite sure why I've included this photo as it doesn't really add anything, but seemed a natural progression in photo taking at the time...
And in this next photo is the finished collar on its stand. Already you can see the points of the collar are nice and crisp, in spite of the fact that this collar has quite an acute angle which would normally be a challenge to get a nice sharp point on the end. This is because of the elimination of one of the seams at this point.And here is the collar finished and attached with its stand to the shirt (although sans sleeves at this point...)
One of the downsides to this method I reckon is the extra fabric requirement as its an awkward pattern piece to work with, but if you're clever you can sort your pattern pieces around it to get the best layout using minimal fabric.
My final verdict? This method works well for just a simple collar on a top that hasn't been "tailored" as such, and expects the upper and lower collars to be cut from the same pattern piece... Will I use it again? If I can be bothered with the extra cutting out of a new pattern piece, and if a crisp sharp collar is a priority, then yes.