In theory this is very simple, but in practice it can be trickier.

The basic requirement is that the torch must be able to supply heat to the job site faster than the work piece can dissipate it.

Two features of the torch used affect this: the flame temperature and the amount of gas being burned. All gasses have a flame temperature hot enough to melt HTS-2000, so the burner design and size tend to be the more important factors. However, the difference between gas/air and gas/oxygen torches is significant.

The work piece also has a big impact on torch choice. Basically small work needs a small torch, and large work needs a larger and/or hotter torch. But this can be affected by the shape of the work. Compact pieces have a small surface/mass ratio and retain heat well, whereas flat work has a large surface to mass ratio and will dissipate heat more readily, making it harder to get up to temperature.

Making a brazing heath from insulating bricks can help to reduce the size of torch needed.

Avoid going too big and hot or you might finish up turning your work into a molten lump!