I didn't realise these would get such a good response, I'll attempt a bit of a tutorial hopefully it will make sense!.
1. Equipment
Tripod, DSLR, Black background, a small table or stool, an off camera flash a remote release, an incense stick and some bluetak.
2.Setup
Place the small table about 12" in front of the black background place the bluetak in the middle and stick the incense stick in it. Setup your tripod and camera, I use an 18-55mm lens set to 50mm and always set the camera in portrait orientation. Hold a magazine behind the insense stick use auto focus to get it sharp then switch to manual to lock the focus, the incense stick should be just out of shot. Set the camera in manual these were taken at f/8 with a 1 second exposure. You'll also need a remote shutter release or use the on camera timer.
3. The pictures
It has to be dark!, night time with the curatins shut and any other light sources removed. Light the incense stick, give it a minute to get going. Hold your flash in you hand behind and below the smoke (point the flash at the ceiling!) then press the remote shutter as you hear the lens open press the manual flash button once. If the smoke trail is a bit naff get an assistant my son lends a hand by lightly wiggling the bluetak base to make the smoke curl etc.
4. Photoshop
Look at the pictures you have taken and choose one that looks interesting (if smoke can be such a thing?). Alter the levels till your happy the background is black and the smoke trail is clear then layers>flatten. Use the snap and the ruler (view>snap/ view>rulers. depending which way you want to mirror you picture drag the ruler exactly to the centre of you image. Create a duplicate layer then edit>tranform>flip horizontal/vertical depending which way your mirroring the image. Select the rectangular marquee tool and select the half of the image that requires removing then hit delete then CTRL+H to remove the guideline. You should now have a mirrored smoke trail flatten the image. The last thing to do is us the gradient fill tool foreground to transparent, select radial 40% opacity and overlay as the blending mode choose any colour you want and fill over the areas you want to colour in.
Practise................:bonk: