If you miss a constituent element of prayer such as ruku` or a prostration and you discover this subtract before making your final tasleem, just add a new rak`ah and recite tashahhud and tasleem. At the end, you should make sujud as-sahaw (prostration for being forgetful). If you discover that immediately after tasleem, you can add a full rak`ah and recite tashahhud and tasleeem in addition to two prostrations for forgetfulness. In case you miss a supererogatory act of prayer such as the middle tashahhud of a prayer consisting of four rak`ahs, you just make two prostrations for forgetfulness.