I've never done the iron transfer ..., But, I have scanned in a picture or design and then printed it on fabric. Then I went ahead and added it to my project.
If I use the freezer paper method I always wash my fabric first so the printer ink adheres to the fabric. Then I iron the fabric onto the freezer paper, trim the leading edge of this, iron it again, put it through the printer, then do a final ironing to set the ink.
Of course you can always buy the sheets of prepared fabric for printers. Just costs more ... does take less time ... and you can't choose your own fabric with this method.
Good luck.
alice