Always Make (At Least One or Two) Test Buttonholes

Always make a test buttonhole before you start stitching on your project.  Buttonholes are usually sewn last, so all your work can go right down the drain if you mess up the buttonholes.  Take some of your leftover fabric scraps from when you cut out the pattern pieces and make yourself a test piece composed of the same elements as the shirt does where you're going to place the buttonholes.  If there's a facing, there's probably a layer of fashion fabric, a layer of interfacing, and another layer of fashion fabric.  If the front is self-faced, you likely have 2 or 3 layers of fashion fabric.

Whichever it is, make yourself a test scrap that's layered just like your project.  You probably don't need more than 3 or 4 square inches...4 or 5 test buttonholes will probably be enough.  Test for thread tension, test for size, test, test, test.  When you're happy with the way they look, go ahead and sew your buttonholes on your project.  Be careful.  It's possible to rip out a poorly done buttonhole, but it isn't easy, nor is it fun.

After all the buttonholes on your project are sewn, then clip them.  I use a seam ripper to cut a small slit it the middle of the button hole, and a sharp embroidery scissors to open it the rest of the way.  Again, be careful.  One, single clipped thread will probably make the whole buttonhole unravel.  I hate when that happens.

Now, press your finger up against the newly clipped buttonhole.  There's usually a couple of fabric fashion threads in there.  Make sure they get snipped off.  Sometimes these can get pulled when you put the buttons in the buttonholes and they can cause a flaw in the fabric of your otherwise perfect(!) project.

Since you've tested it, you know your  button fits and you're all set.