This was done very quickly to show you my approach, so it's not perfect.

The advantage to doing it post is that you can use real or realistic flames.

The disadvantage is that you have to really experiment, which means a lot of test rendering. The lighting is important because flames produce light. Unless you are doing a fantasy/magic scene like this one is supposed to be, your flames will produce lights and shadows. So you have to get that right.

Using a prop based flame has its own advantages and disadvantages. You will have to decide which way to go. I personally like to do it post. I have problems with the material room. I know enough to get in trouble.

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.