You cast red instant cards and red sorcery cards from your graveyard as part of the resolution of Chandra Ablaze's third ability. You don't choose which ones to cast until you're actually doing so as the ability resolves. You cast only the ones you want to, and you may cast them in any order. Timing restrictions based on the card's type (if it's a sorcery) are ignored. Other restrictions are not (such as "Cast [this card] only during combat"). Each card you cast this way is put on the stack, then the ability finishes resolving. Those spells will then resolve as normal, one at a time, in the opposite order that they were put on the stack. They'll go back to the graveyard as they resolve.