Over the years I have tried many ways to press hems, from pinning, marking, using small metal gauges but always ended up with sore, hot fingers & if a narrow hem, it wasn't always neat & even.
This tip works for me & is especially helpful when needing to press a 1/4" (5mm) hem.
1. Stitch a line at exact measurement of where you will do the first turn.
2. The line of stitching makes it easy to roll the hem over & press with the iron without scorching my fingertips.
3. Now roll again using the first pressed edge as a guide & press the hem.
4. Stitch hem in place -this may be with a straight line or a fancier stitch to add an interesting finish.
The folded edge gives a guide for placing hem under machine foot to ensure stitch line is consistently even from edge of fabric.
(I love using my new Osilo Pro Smart Iron, check them out at The Pickledish Patch )