Board Thread:General Discussion/@comment-26342775-20150914132551/@comment-24674051-20150914152004

no need to know ice magic. slight adjustments of a glamour spell, plus some good acting, can achieve the same effect. and we know that Zelena had the Clover charm, so the glamour spell was a piece of cake. And if she casts the glamour spell from the charm, and then adds the frozen effects with her magic, then her magical cosmetic additions would be canceled by crossing the town line, but the initial glamour would remain intact.

Just supposition, but that's how I'd do it.