Q: Until recently I thought I did not have to give zakat because I was under the assumption there was only one nisab, the nisab for gold, and I have never owned more than the nisab for gold. However now I am aware that there is a nisab for silver, and that this should be used when calculating zakat. Since I haven't paid zakat until now, I would like to start calculating my zakat for the previous years as well as for this year however I have some questions as I haven't calculated zakat before:
1) Do I estimate how much zakatable assets I have at the END of each zakat year and then pay 2.5% of that in zakat?
2) The nisab value constantly changes, for example at the moment the nisab for is around £294, but previous years it was different. So when determining if I own above the nisab in previous years and am therefore eligible for paying zakat, since I don't know what the nisab was in those years should I use the current nisab when calculating for previous years?
3) Since I did not know I had to pay zakat, I also have not taken part in qurbani (udhiya). How should I go about calculating how much I owe for this?
4) Can I pay my zakat through an islamic charitable organisation, such as MuslimAid, IslamicRelief, etc?
Forgive me for the long question, but I want to make sure i'm paying zakat correctly, especially as I had been ignorant of it for many years. May Allah subhanahu wa ta'ala make it easy for us to fulfil our religious obligations.