I read about the blackout method but doesn't this adversely affect the fish and plants? And if not, do you leave the filter running?
A blackout will not adversely affect the fish (though plants may get leggy with a longer blackout).
You should keep the filter running.
Also I have red cherry shrimp, amanos and and three kind of snails and don't want to lose them.
Be sure to keep ample surface agitation if you are going to attempt the blackout.
There are various methods of fertilization; for example, Estimative Index (EI) or PPS-Pro will work. For more information, take a look at my Primer to the Planted Aquarium (linked in my signature below).
I am also not clear on how to use fertilizers with the shrimp in the tank.