The wash I use is made of the following stuff: artists' watercolor(the one in a tube), water, dishwashing liquid, & vinegar. The watercolor are very forgiving. If you're not happy with the wash that you've done you could always put your model under the tap & you're back to square one. Sepia color is my 'jack-of-all-trades' when it comes to washes.
The diswashing liquid helps in breaking the water surface tension, without it the liquid will tend to regroup itself into droplets over your painted model. A drop would be enough.
The vinegar helps to reduce the tendency of washes to leave hard edged demarcations once it dries. A couple of drops will be enough.
Then you'll need a container & a soft brush.
The mix: just a small amount of paint on the side of the container & slowly add it to the water/vinegar/dishwashing liquid solution.
I usually do a 'slap-it-all-over' application then I place my model over a paper towel to drain the excess wash. The one advantage of using this type of wash is that after you apply it & dried & you find that some areas are a bit darker you could always use a clean,soft, flat brush & a bit of water (or even saliva)

to remove the excess(something impossible to do with dried acrylic or enamel based washes).
Hope this helps.