Wintertodt only requires 50 firemaking, but you should get 2 warm items and a steel axe before starting it on a Zeah Ironman.

The easiest warm items to obtain are the clue hunter garb (found near Shayzien) and a staff of fire (bought in Arceuus with 20% favour).
It is also recommended to unlock the Arceuus spellbook and enter Tithe Farm for herb run teleports.

Hover each item to see its name.

Solo Wintertodt is better construction XP, but worse firemaking XP.
If you're under 60 agility, you should always solo. Otherwise, go to world 309 and use the shortcut in between games for agility XP.