Carefully? J/K, I avoid heels over 3".
But for high heels between 4 to 6 inches -- go with 1 full shoe size larger. Also decide if you want open or closed toed shoes, and practice standing in them to see how well you adjust. About 15 minutes a day should do it. Once you have that down, you can practice walking and working up to longer stretches at a time.
It can be a pretty big adjustment for your feet, so make sure you give them a break and go with flats in-between. Practice walking and standing at night, too, when your feet have swelled more after being on them all day.