How do I stabilize large hand-embroidered piece?

I'm currently working on a large hand-embroidered motif for a costume, and I would like to iron it to smooth it out and stabilize, but I'm afraid that the steam will shrink the embroidery floss and wreck the work. I've pre-washed the cotton body material, so I can steam it no problem, but will the pattern be ok? Could I iron some interfacing on the wrong side once I'm finished?
Yes, I plan on lining the piece after I'm done.

The best way to keep it flat while sewing is by using an embroidery hoop. If you steam it GENTLY from the back once it is complete, the embroidery should come to no harm.

I would be very wary of using a fusible interfacing or stabilizer on the back when it is finished. To protect the back when the garment is in use, line it.

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