It’s important to create a barrier between the hot iron and the vinyl itself; in one of my tests, I allowed the vinyl to get too close to the heat, and it puckered a little bit like if you were to touch flame to a piece of plastic. The best reviews I read recommended using sheets of plain printer paper in between the iron and the vinyl (five sheets thick worked well), and to that point, I found that the paper was transparent enough that you could start with a single sheet over the graphic (just to see through and ensure that none of your details were shifting around). Once I could tell that the graphics were held in position under one sheet of paper, I was gave the paper a quick tap with the tip of the iron (no steam!) to begin to bond the vinyl, and then laid the other 4 pieces of paper on top before ironing over it for 15 seconds to affix it completely.
For larger graphics, I found it helpful to bond the fabric and graphic in stages, and not try and do it all with one swoop of the iron.