Remember that you are still creating a painting. Try to focus on using brushstrokes to create lifelike features and colors to add depth and dimension, instead of panicking about whether or not you made the space between your eyes the exact distance. Focus more on the physical components of the painting, as you would with other paintings, as opposed to perfecting the physical components of your own body and face.
While you could stare into a mirror as you create your self-portrait, it will be a bit easier if you utilize a camera. Take a picture of yourself and then upload it to a website or order prints so that you can just look at the photograph. Place the photograph somewhere where you can easily see it since you will be glancing at it a lot. On top of the easel would be appropriate.
Chances are, unlike the foundation aisle at the local drugstore, your paint palette is not going to have exact colors to match your skin tone. Use a palette to mix colors together until you find the correct shade. Even flesh color that comes in a tube may very well not work out. You might have to do some mixing of colors with your hair, especially if you have highlights or some grays.
Consider putting yourself in an environment that has some sort of meaning to you. If you took a photograph at your favorite restaurant, the library or a zoo, consider adding those details to the picture based on the camera image. Create your own backdrop, whether it is the studio where you are creating the painting or a fantasy world where you are surrounded by images from some of your most bizarre dreams.