This can be difficult because it’s awkward to step outside of oneself and describe ourselves. In third person subjective and first person narrative voices, we follow the main character along and share in his or her thoughts at the story progresses. But how can we reveal important details about our main character or describe him or her without sounding awkward or forced? Many writers use revealing devices such as having their character stand in front of a mirror, but this is an over worn cliché that you should avoid. You can give bits of description through the dialogue of other characters. What do they say about the character or how do they react to him or her? You could also reveal a character's emotional state through a description of the weather.
Many writers find the single viewpoint narrator makes for cleaner and more engaging writing. The reader gets invested in the main character’s perspective and follows the story more closely rather than meandering from viewpoint to viewpoint of different characters. Still, if you like writing from multiple viewpoints, you’re not alone. But it’s a good idea to limit one viewpoint per chapter rather than include two viewpoints within one chapter. That can confuse the reader and also break their reader’s trance—interrupting the flow of the story.