The best way, in my experience, is to glue fume the plastic baggie and then dust with black powder or magnetic black powder. Then you photograph the print(s) and then lift them with fingerprint tape and place them on white backing cards.
Sometimes it is a challenge to lift the fingerprint with tape from the flexible surface of a baggie, so what I do is use an embroidery hoop to secure the baggie before making the lift (it also helps in photographing the developed print before lifting—it takes out the wrinkles in the baggie that make photography difficult.)
Here is a photograph of a plastic baggie secured with an embroidery hoop.
If you want more information on glue fuming and dusting, check these links:
We always had great results using super glue fuming in a vacuum chamber. Ten we apply yellow dye stain. Once the stain is dry examine it under a forensic light source to see the prints then photograph them.
manifest the latent print on plastic bag, 1, UV light source orienting reflex is ok to manifest the latent print on plastic bag; 2, 445wavelength uniform light source is OK, too. and the color of the plastic bag is accordingly, more question pls feel free to contact me by firstname.lastname@example.org I am a crime scene light source device manufacturer from China, sorry for my poor English.