You probably will get as many different methods as answers here. I like to paint the base color or colors first. Then after a day or two to dry I will apply white was using a smaller (#1 or #3) paintbrush and hand brush a flat white paint, preferably an enamel (Humbrol or Testors) as they cover and handbrush better. This method gives a good replication of the scruffiness seen on most winter camos. Remember, winter whitewash is a “temporary” coating applied in the field, usually by the crew, with basic tools in a quick manner. Not a factory paint applied under perfect conditions by spraygun.
Winter camo was field applied with a thin white wash using what ever they had available to apply…spray, mop, rags, brush or bush branches dipped into the paint. Sometimes it was applied as a blanket cover, other times as a disruptive pattern.
When the King Tiger first appeared, it came from the factory in overall Dark Yellow and was camouflaged at the unit level with the Olive Green and Red Brown. Late in 1944, factory camouflage of the same three colors began to be applied. The Whitewash was applied at the unit in the field when the snows came. So really the choices are all yours for patterns in both cases.
Like the others have said, there are many different ways, but I like to put in the bottom color first. Here’s a description of how I did it on a StugIII, has in progress pictures if you are interested.