|Another shirt-making tip/technique which I think I created, but it's probably already out there somewhere.
Patch pockets usually have you fold over and iron flat, three sides before attaching to the shirt (the fourth, top side having been self-faced). Instead of making marks with pens, markers, pins and/or rulers and risk burning your fingers with the iron, I did this.
I cut a thin piece of cardboard, from a box of tissues, and carefully measured an exact 1/2 inch (I have another for an exact 1 centimetre) and cut that cardboard strip (about 4 inches long). Now place the exact cardboard strip near the edge of the pocket to be folded+ironed. Nudge the fabric over the strip until it lines up with the edge of the strip. Leave it there and iron on top. Pull cardboard strip out. Voila, a straight ironed edge the exact width. Move to the another side of the pocket - this is especially good for pointed lower hems, and repeat. Repeat for sides. Turn pocket over to right-side up, and quickly press.
There you have a perfectly ironed patch pocket with neat straight lines, all with the correct width turned under. Place on your shirt and edge-stitch into place.
This also works for the slightly curved edge of the uninterfaced side of a collar band, where you have to fold+press under the seam allowance before making the 'sandwich' (interfaced side of band, uninterfaced side of collar, interfaced side of collar, uninterfaced side of band on top). It works for me! Just move your cardboard strip around bit by bit with the iron - it'll cope with slight curves.
Hope that helps.