There are a number of ways in which photographic self-portraits can be achieved. The simplest is to look directly into a clean mirror, raise a camera to your eye and release the shutter. Remember to focus on the reflection rather than the edge of the mirror. With care, good composition and a suitable background this can be worthwhile.
Another approach is to set a camera up on a tripod and use a wireless or infrared remote release (or a cable concealed beneath a carpet or inside clothing). Camera alignment can be achieved by tacking small pieces of masking tape onto a wall or screen behind your proposed location so as to define the field of view. If a chair is required position it well within these limits and set the focus by measurement. A small aperture provides sufficient depth of field to give a margin for error. Once satisfied, remove the tape from the wall before adopting a position and releasing the shutter.
A self-timer can also be used although it may not release the shutter at the ideal moment. Some cameras indicate the approach of shutter release with a light or beeper. Nevertheless, adopting the desired pose and expression at precisely the right moment can still be challenging.