Ah. Million and one possibilities.
I'd strip it off, steam it for a good long time while bending it *gently* against the warp (don't burn* yourself) then pin it down for at least 24 hours. Let it sit for a few hours after you unpin it, to see if takes up the warp again. If it does, repeat.
Definitely shrink the tissue on both sides at the same time, simple 1/16" sheet outlines don't have much warp resistance.
If it absolutely refuses to flatten after two or three goes use the existing parts as a template and cut new from the sheet scrap in the kit. Another possibility is a laminated outline, which will have a better strength to weight ratio and is likely to be lighter than the sheet version, but we can leave that for another day.
I have been known to pre-shrink tissue before applying it to flat, lightweight frameworks. Take something like an old picture frame, it needs to be *very* rigid, tape the tissue to that, mist or steam shrink it. Use the ready shrunk tissue to cover the framework, with as few wrinkles as possible, before doping it. It'll shrink a bit more if your "dope" is a shrinking dope, but not too aggressively, taking up any small wrinkles.
*Easier said than done.