How to perform dNBR in SNAP

Hello i want to perform dNBR using sentinel2 how do i perform it in snap?i have only performed NDVI both for pre and post fire images now i want to determine burnt severity levels between both pre and post fire images how do i go about it?

  1. Right-click on the first product, select band maths.
  2. Enter the equation for the normalized burn ratio: (B08 - B12) / (B08 + B12) to get NBR1
  3. Repeat for the second image to get NBR2
  4. Calculate their difference: NBR1-NBR2
  5. Use these values as indicators for burn severity: