I haven't used it, but I got a book of iron on transfer quilt labels for Christmas. In the front it has directions, and what I'm about to write comes straight out of those, but has not been tested by me, so I can't vouch for it. And of course if it is a different maker, the temp, etc. might be different.
It also suggests doing a test pattern first (and small test patterns are included on each page) because irons and fabrics vary, and protecting the ironing board surface with a piece of clean fabric or heavy brown paper. Use light colored fabric that has been washed and pressed (and it says for washability that polyester or poly cotton is better than 100% cotton, and 100% cotton will fade substantially)
1. Place fabric right side up, with transfer ink side down.
2. Cover design area with a paper towel or tissue paper. Preheat dry iron to between wool and cotton settings. Place hot iron on towel and hold down for 10 to 15 seconds. Lift iron straight up and down to all areas of transfer; do no move iron back and forth. Replace iron several times so area under steam holes is transferred.
3. Carefully lift one corner of transfer to check that design has printed. Do not remove completely unless you are satisfied with the results, as it is impossible to realign the transfer once it has been removed. Repeat step two until transfer is complete.
I hope that helps. My book is "Iron On Transfers for Quilt Labels", by Barbara Baatz, put out by American School of Needlework (and also it lists Clotilde.com on there somewhere)