Safely? Best suggestion from me would be to use enough ND filter and then use live view to focus. I shot the transit with a 9 stop filter stacked on an 8 stop. I then used live view to focus on venus. I would not look through the view finder because I think there are just too many bad wave lengths being focused that the ND filter doesn't affect. DOF had the sun slightly out of focus. I was planning to do a sun focus too but clouds covered me up after my third shot. I got about a three minute window to make a shot.. )-:} Shot with D800E in Live view, Nikon 80-400mm VR lens at 400mm, ISO 400, F10, 1/500s.
The best shot I saw (not mine) used a single 8 or 9 stop filter, ISO 50, 1/8000s, don't recall f stop or lens. Shot was very nice though.
I know this info won't do anyone any good for a venus transit but it still might be fun to play with imaging sun spots and such.