IMHO a pickled finish looks best on oak, where you have a pronounced grain. Otherwise you will be doing a faux finish, which is ok, but different.
For a pickeled finish, you start with a white paint and wipe most of it off, leaving it in the pores. When you get the effect you want, let it dry well and coat it with a clear finish. I use Varathane’s Diamond Finish because it dries clear and hard.
If a customer wanted me to do a pickeled finish over a dark stain, I would first strip it all off with a chemical stripper, and then proceed. If you don’t, you will have difficulty covering teh dark color just right.
Now a faux finish is another story, and I’m no expert on that; you can usually find classes on that at paint stores.