Give any loose threads hanging off your hem a quick trim before you stitch the hem up as they will poke out and just generally be in the way.
With an iron, press the bottom edge upwards inside the shirt by a foot width (6mm). You can make your hem as deep as you want, its personal preference however do remember, the more curved your hem, the narrower your hem allowance needs to be.
It is almost impossible to sew a deep hem on a curved piece.
If your fabric is thick, you can slightly clip the corner of the placket off so it's not too thick to fold over.
Press the hem up once more, the same width as the first fold. This techniques is commonly referred to as a double turned hem. Pin it in place.
Edgestitch the hem down in place. Generally speaking you should do this from the topside, but if your bobbin thread / tension is well matched, You can do this from the inside - as it's much easier to get a flat even finish.
ALTERNATIVE HEM METHOD
There is a faster method to finishing the hem off. However it is definitely less polished. It would never be a technique used on a classic Mens business shirt, but is sometimes found on Women's blouses bought from High Street stores like H&M and the like.
Overlock the edge of the hem, letting the blade cut 6mm off as you overlock around the whole shirt hem edge.
Press the overlocked edge up once (6mm), pin in place and edgestitch it down. It's a very straightforward method.