Generally u allways want to get both Procs (heating up and Pyro) before doing something special.
Your normal rota is:
1. Fireball until heating up
2. When Heating up, fireball and infero blast.
3. When Heating up and Pyro, use fireball and then pyro instantly (if both up again just repeat)
4. If u just have Pyro, continue fireballing until both proc, then do step 3.
For big Ignite:
Set up a Heating Up + Pyro Proc, cast another Fireball and use Pyro. You most likely will get another Pyro, use it instantly (this means no fireball in between) if u are decently lucky u get anoth