I learned alot by cutting some scrap pieces off a bag, and practicing with my iron. The stuff melts VERY easily, it doesn't need much, and it's very easy to over-do it.
I found that proper technique is more important than perfect heat.
I used a 1.5" x 2.5" piece of wood that was a support piece from a busted chair ( was PERFECT for this,IMO). I folded the food-filled mylar bag tightly over the wood,squeezing out most of the internal air, then ran a very hot,standard clothes iron over the mylar, then back again.
Anything more was un-necessary, and would over-melt the mylar.
Another CRITICAL aspect was ensuring that there were no ripples where the mylar was folded over the piece of wood. Make sure everything is clean,smooth, and tight. A second pair of hands helps, especially at first ( till you get the hang of it).
As IlliniWarrior said above, try to remove any particles or dust from the inner part of the bag where the seal will be. I would just wipe the area with a paper towel. This is most critical with finer stuff like dehydrated potatoes or flour, not quite so much with more solid stuff like wheat berries,beans or rice.
Hope this was helpful, holler back if you have any questions........
One more point, try very hard to NOT let the iron touch the bag anywhere you don't intend to melt and seal. If it touches the main portion of the bag, that has air and/or food behind it, the iron will almost instantly melt through the bag, leaving a hole.You want the iron in contact ONLY with mylar that is supported by the wood underneath.