If you want to do streaks like my models, it just takes a steady hand and patience.
I don't use a fine detail brush, I use a size 1 brush with a good tip. Having a good tip is the key, your streak will only be as sharp as the blob of paint on the tip of your brush. So you need the brush to have a good tip, and don't load up the brush tip too much, you can "roll" the tip of the brush through the paint to create a good tip and if you have excess paint on the brush, wipe it off on something instead of going straight to the model.
Some people like to do the streak from the base to the tip, I personally prefer starting from the tip and going to the base, so I line the brush up along where I want the streak to be, touch the tip to the model to create the tip of the streak, then pull downward to the edge of the armour plate (the fat end of the streak) and apply more or less pressure depending on how thick I want the streak.
Apply streaks in multiple layers, so for mine I base coat the armour plate a very dark blue (almost black) then do 1 layer of large streaks with a mid-blue then another layer of smaller streaks with a light blue. If I weren't lazy, I would probably do a final layer of tiny streaks just right at the edge of the armour plates with a very light blue (almost white).
It is time consuming if you want to create good looking streaks. That's why the streaks on my Trygon looks better than the streaks on random Hormaguant number 42... I didn't want to spend as much time on the smaller models so the streaks ended up messier.