What is a safe way to tan? My answer is none; however, I understand that most of my patients want to get at least a little bit of a tan. So the advice I give them is the same advice I give my psoriasis and my Vitiligo patients, where natural sun is known to help their disease.
Going into the sun before 10 am or after 4 pm for 20 minutes and still wearing a very light sunscreen can make sure that you get a baseline tan without forming a sunburn. Instead of trying to get tan in just one session, try to split it up into several sessions, such that you're not in the sun for a prolonged period of time.
I highly recommend avoiding tanning booths, as the types of bulbs that they use, the amount of energy you're getting, and the amount of UV light you're getting is not regulated. There's no way for us to know what they may lead to in the future.
If you're in the sun, and you notice that your skin is getting bright red, or you're noticing pain or blister formation, that's when you know that you've gone too far. You need to get out the sun. Avery subtle pink foe 10 minutes is all that's needed to form a baseline tan. Doing this three times a week can make sure that you can form a baseline tan without getting a burn.