You want the fire inside the text ?
Staring with a base layer (you can always delete it afterwards - makes visibility easier)
A text layer above that .
The flames layer above that.
1) In the text layer, make the text a selection Layers -> Transparency -> Alpha to Selection
2) On to the fire layer and add a layer mask Layer -> Mask -> Add layer Mask and use the Selection option.
and thats it....example 30 second animation: https://i.imgur.com/qVVPorI.mp4
Staring with a base layer (you can always delete it afterwards - makes visibility easier)
A text layer above that .
The flames layer above that.
1) In the text layer, make the text a selection Layers -> Transparency -> Alpha to Selection
2) On to the fire layer and add a layer mask Layer -> Mask -> Add layer Mask and use the Selection option.
and thats it....example 30 second animation: https://i.imgur.com/qVVPorI.mp4