(BTW - My fashion industry friend did exactly the same thing when we were sewing yesterday. We laughed about the habits of industry-trained sewing types who - no matter what the fabric is - always have the iron set on linen and high steam. Good to know I'm not alone in the steam the bejaysus out of everything camp!!).
If you use good quality woven interfacing IT CAN BE FIXED!! .... a bit. (If this happens with non-woven interfacing you're in a spot of trouble. It's hard to resurrect shrunken synthetic fibres).
Step 1. Turn your iron back to COTTON setting.
Step 2. Press and STRETCH out the fabric to flatten out the puckers and wrinkles. Press down REALLY HARD with the iron. Smooth it with your hands (as much as you can without burning them on the hot fabric) and hold it flat until it cools.
The pic below shows the same piece still on the (sleeve) ironing board - nice and flat.
Step 3. Once you take the fabric off the ironing board and move it around you might still see small ripples, but it won't be too bad - not visible on most fabrics from a few feet away. (Unfortunatley most people look at my samples REALLY CLOSELY!!!).
Treat the bag kindly and press it back into shape as required. It'll always be prone to a bit of wrinkling.
NOW.... This is what I SHOULD have done before I started the bag....What is it that I tell everyone who asks me an interfacing question.....? TEST IT ON A SCRAP OF FABRIC FIRST!!!!
HINDSIGHT IS A WONDERFUL THING.... If I'd tested a few options I would have discovered that the Medium-heavy interfacing (above) was the one I needed. It would have given the fabric the structure I wanted and it can take any amount of heat (in fact it NEEDS a super-duper hot iron to set it!). This particular bag needed lots of pressing and I should have thought that out before I put in an interfacing that likes very LITTLE pressing.... Having chosen the wrong interfacing, I wish I'd checked that the iron was on an appropriate setting.... and I wish I'd had a manicure before I took that photo....
Oh no - but I thought I knew it all...
I have a quote from the fabulously ironic Jane Austen pinned to the wall (above the iron!) in my studio. "Why not seize the pleasure at once? How often is happiness destroyed by preparation, foolish preparation!.."
Vlieseline have a search function on their website to help you choose the right interfacing for the job. A bit of research and a bit of testing can save a lot of pain. But in the absence of forethought, please refer to the tutorial, above....