Washes 101: Well,first of all...what kind of paint did you start out with?Is it acrylics or enamels?You want to stick with the same kind of paint,it alleviates compatibility issues,believe me you don't want that headache,but if memory serves...I think you can do an enamel wash over an acrylic paint base.I'm not sure how well acrylic washes work out?I know acrylics are the "new wave thing" but I'm old school and still like to use enamels.
You can adjust it to your liking,but basically you want to use a ratio for a wash like 10:1.10 parts thinner/reducer to 1 part paint(I work in drops). If you adjust it for more thinner to paint,then you will get a thicker/darker wash and its fun to experiment sometimes,but I wouldn't do it on a Salzo X-Wing.
You can apply the wash by loading your brush and just letting it run where you want it(let chance and fate take over),or another way is(and I do this) is start with just a brush loaded with plain thinner(a good ammount)paint it on as if you were painting the way you want the streak to go,then just touch your brush tip and watch the dark color bleed and run where you want it.
You can apply multiple washes and in different shades of light to really dark grey to jet black.For greese semi gloss back is good to use,but you can use flat as well.The semi-gloss is good for scale,and the smaller the scale the flatter you want it to go.Its a learning process and the more you do it,the better you will get.You'll find what works for you and what techniques you like.Before attacking your X-Wing,practice on a junker model or scrap styrene.Good luck!