One possible key reference element in optical alignment is represented by the rotational stage, a mechanical bearing, or any similar suitable device having enough accuracy and precision so that optical tolerances are reasonably relaxed wrt imperfections in the rotational movement. This allows a safe, reliable, easy to reproduce, determination of both rays parallel to the axis or to their centering within almost any plane. An image derotator, that in its simplest form is made up by three flat mirrors arranged in a so called K-mirror layout, moving together on a precision rotating stage, seems to be the most safe, strong, and self built-in alignment tool. Moreover you can use the mechanical part as well as the optical one. Care has to be given when internally and externally aligning has to be accomplished within a certain degree of precision. To further make the situation more complex, the technical overall requirements can be tight enough that the distribution of the error budget among the various components (imperfect mechanical rotation, imperfect internal alignment, flexures during rotations) is not due to a single item. In this case, in fact, a number of tips and tricks can be useful to find out which is the best approach to follow. The specific case of the two K-mirrors on board LINCNIRVANA is here illustrated in a few lessons.