Well, what is safe hinges on your definition of the word 'safe'. So, I won't even go into that, just do some searches, and see if it is as 'safe' as you would like it to be.
As for dosage, if all you are doing is winny, I would say start at 50mg/ed. You should start seeing results in about 2 weeks. I always notice winny in 10 days.
Take it for 6-8 weeks for a start.
As for precautions, well, read my first sentence. I am not here to spoonfeed you info. Do some searches. The amount of things on this site alone about winny is staggering. And I am not going to spend the next hour and a half typing them all out for you, just so you can ask a question that would have been answered by your search.
Good luck on your search, and good luck on your cycle. Let us know how it goes.