Cheers Carl and Leon,
The way I do this is first do a test strip with an 00 filter. When you have a suitable time (say 10 secs), do another test strip with #5 filter with roughly the same time (10 secs), then, under safe light, switch back to the 00 filter. Diffuse for the same amount of time (10 secs). For the final print, tweak the times to suit, keeping the diffuser moving below the lens. The closer to the lens, the more diffusion.
For a diffuser, I use a clear piece of plastic, the type you'd find on binders or in cheap picture frames, sprayed with hairspray. Spray a layer, then leave to dry, and spray again until happy. It dries quickly and can be washed off easily. The trick is to get the times right so the blacks bleed nicely, creating halos if you want them. Contrasty images are best.
Here's one (shot in Kathmandu) that shows an extreme reaction to diffusion.