Start with good light and a fair amount of it. Smartphones, even expensive ones like the $1,000 iPhone X, all use very cheap $15 cameras. These cameras do not do well in low light situations, so you should stay away from low-light areas such as inside buildings or in the shade. One of the best light sources is the Sun. It's free and predictable, and if you go out an hour before/after sunrise or sunset, the light is often very beautiful and will add a lot to the quality of your selfie.
Now that you've got the light, you need a scene. Look for an area with a very clean background. Avoid busy backgrounds. Try to find an area where you can stand with the Sun, low in the horizon, so that it's coming right in your face. Try photos with the Sun behind you, to the side and at about a 45 degree angle to your face. The light, being at different angles, will produce a different looks one of which you'll like more than others.
Use a selfie stick. Smartphones are all equipped with wide-angle lenses which are produce a great deal of distortion and why they are horrible for making portraits. Fortunately, your iPhone X has a telephoto lens which produces significantly less distortion (if any). Because of the longer focal length, the iPhone X lens won't stretch people's faces but instead will compress their facial features a little bit. This is actually a very, very good thing as compressing the perspective on portraits will often make people look MORE attractive than they do in real life. However, this means using a longer focal length, which means you won't be able to hand-hold the camera. The solution is to use as long of a selfie stick that you can get, or set the phone up and use a self timer.