Writing lyrics first might give a more solid base to hold on to when going into writing the music. As for "framing" it... you're giving yourself restrictions as to stuff will fit in.
However, if you're an experienced "framer" you might pull it off fairly easily. It requires some practice. When just starting out framing might be the easy way, but not having anything fit in might leave you frustrated.
So I would advice to just write. See how it all fits in, see what parts could eventually be changed.