I just did this!
Here are some tips:
1)Cut a hole in a plastic grocery bag and put the throttle cable through that. This should prevent any lube from going onto your bike.
2)Hold the cable luber in a rag or some paper towels to soak up the excess lube. Also put some paper towels on the other end of the cables to prevent the lube from running all down your bike.
3)The throttle cables have slits in the black insulation towards the end of the length of the cable. In hindsight, I should have taped these slits over to allow more lube to go through instead of spilling out and getting soaked up by the paper towel.