love Katy ***
I think that they passed X-rays through crystallised DNA onto photographic film (or something similar) and were able to see the diffraction patterns (hence the name).
As far as I know, one photographic plate showed a cross, surrounded by a faint circle. Not entirely sure about this, but I think that they (Watson and Crick, I've just remembered) built a model out of aluminium pieces, that would have given the same diffraction pattern. (if it was about 1000000 times smaller of course).
Hope this helps.