I am also not good at applique so I have tried preparing my pieces like this and am getting better:
I make my templates out of watercolor paper you can get it at any hobby/craft/art store. It is very heavy weight like card stock. I cut out my fabric with a very generous allowance, sometimes up to a half inch. I use the card stock template to press my edges under as heat from the iron won't affect it. Once my piece is prepared I remove the template then use Arlene's Tacky glue to glue the turned under edge in place. Also around curves and such I make sure I notch or clip the seam allowance so the fabric curves smoother. I still do not get the amazing results I see here but I am getting better. The glue helps a lot and will wash out and I can prepare pieces well in advance and not worry about my pressed under seam allowance losing that crisp edge.