BBorBust
If you make your own, what is your process?
Well, back in the Dark Ages when Tanker an I were "young," there were no pre-packaged washes. I remember when model-specific water-base acrylics first started showing up in LHS. (And, that was largely PolyS for the railroaders, to start.)
You have to alternate paint chemistries, and/or use artis oils thinned with turps or odorless spirits.
Oils were very much like modle paint, much higher thinner to paint volumes.
We did things now considered obsqure and odd. Like use actylics to paint a model, then heavily-thinned enamels to wash details in. Or vice versa.
You did not want to use the same thinner for your wash as for the model paint, lest if lift both.
It's now canon to used clear coats between finish paint and washes, not so much back then. Which probably reflected the quality/thin-thickness of clear coats "back then."
So, really, there's no "formula" per se, so much as there's just practicing. Which is what cheap kits are for.