There aren't many tutorials but here's what I usually do.
First, pick a normal circular brush with 0 hardness, size 200, and of opacity 15%. Start by brighten the area you want to apply your smoke. Don't lighten it too much tough.
Once you are done, create a new layer. And set your brush to 25% hardness, size 100, and opacity of 35%. Start by giving it some smoke brush by brush. There's a technique to give your smoke a 3D feel. Click and hold and make an initial shape for the "cloud". Once you are satisfied, brush it over (but over a smaller area). So on and so forth.
You then continue to give greater detail to the smoke by lower the size of the brush and increasing the opacity ( max recommended is 80% with a brush of size 30 for a 1600 X 1200 resolution screen)
There are no definite way to do it. I have a few techniques, it really depends on the screen shot, the angle, and my mood
