Grasp horizontal bar (lower than chest height) with hands shoulder width or slightly narrower. Position forefeet back from bar with arms and body straight. Arms should be perpendicular to body.
Keeping body straight, lower chest to bar by bending arms. Push body up until arms are extended. Repeat.
Both upper and lower body must be kept straight throughout movement. Height of bar affects difficulty of movement. See Gravity Vectors for greater understanding of how body angle influences resistance. Also see wider grip push-up.
Resistance can be reduced by performing exercise on higher bar or edge of platform.
Resistance can be increased by lowering bar or eventually performing close grip push-ups. If next lower bar position is too difficult, place feet on platform, fraction of height as difference between bar height increments.