Here's what I learned:
I was working with 100% linen (a scrap, don't be silly) and a printed picture from my computer- the linen had not been previously laundered so it still had any size that the manufacturing process added. I traced the picture heavily as recommended and lightly as a test and pressed for the required 5 seconds on the linen setting (says cotton but I was working with linen so I went hotter).
The transfer came out very well except for a couple of holes... where my iron has steam vents. So, they tell you don't move the iron, just press? Ok, but press twice, repositioning the iron carefully so the transfer doesn't move. Also the lightly marked transfer came out pretty well too.
Out of curiosity, I washed it... and the light transfer came out entirely and the heavily marked transfer came out mostly- it did lighten up a LOT. I did wash it in hot with Dawn dishwashing liquid (by hand- scrap, remember?) and I did wash it almost as soon as I had looked at the marks so I don't know if they would set in more over time. I'm inclined to think probably not.
For myself, I'm going to mark lightly, press twice moving the iron to cover the steam vents and make sure I get the whole transfer and once I am done, wash in hot water to remove the markings. I did not use the transfer paper that is sold to go with these.
Again, this is on linen- I'd have to test it on cotton before I used it to mark a quilt.
Aunt Martha's Hot Iron Transfer Pencils Joann's claims that they are only available on line but I found them at my local store.