Hi Nuno,
I think I may have wrote the other thread you are referring to about adding in the white light visual. To be honest, my basic starting point with pranayama was as David suggested, to feel the sensation physically going up and down the spine, I still do this in every session. In that other post I was suggesting adding in the white light on top of this. It's important to be careful here as you can over do it with this white light stuff and if it is confusing then don't bother with it.
When I started I had a very difficult time feeling the spine as I brought my attention up and down. As time went on and after a lot of twice a day practice sessions, I started feeling more and more of my spine and more often. I had days when I couldn't feel much but overall there was a steady progress over several months.
I still have days when I "feel" or see less and others where there is more, the important thing is to continue practicing because that is the foundation for both progress and achieving any milestones along the way. If you look back at a long period of time like several weeks or months, you will be able to see steady upward progress. If you look too closely for progress on a daily basis you will be more aware of the "two steps forward, one step backwards" adage.
Hope that helps,
Anthem11