How do I whitewash knotty pine boards previously stained?

You probably should sand first. Then paint with a solution of paint and water. Try it on a board (left over) to find the right look for you.

Can you turn the boards over and use the unfinished backside? Otherwise, I would take them to a cabinet maker to run through his planer, or get a good sander and sand them down starting with roughest grit 60-80 and work up to 150. Then mix paint and water, and run the mix into the wood.

I agree with Aljosjer. Just lighly sand to remove any gloss. Thin down the paint 50-50 with water. Dip a paint brush in the paint, paint the wood, and remove some of the paint with a cloth. Want more white wash remove less paint,want less remove more. With a little trial and error you can get the effect you want. Try on a sample board or in a inconspicuous area.

Best is to sand back first so that the old stain doesnt bleed through

And then for the whitewashing, I think this will help- https://www.youtube.com/watch?v=IuyujnYvkP4

thanks. can I use a second coat of whitewash in a light turquoise colour after the first coat of whitewash in white? My feature wall (kitchen and living room facing each other) are in Ocean Breeze (a light turquoise) - I think it would be nice if there was a little touch of turquoise on the planks. what do you think?