If I practice tranquility meditation how would I go about developing insight.How does a samathayanika realise Nibbanna.
As per the Visuddhimagga:
Herein, “purification of view” is the correct seeing of mentality-materiality.
One who wants to accomplish this, if, firstly, his vehicle is serenity, should emerge from any fine-material or immaterial jhāna, except the base consisting of neither perception nor non-perception, and he should discern, according to characteristic, function, etc., the jhāna factors consisting of applied thought, etc., and the states associated with them, [that is, feeling, perception, and so on].
When he has done so, all that should be defined as “mentality” (nāma) in the sense of bending (namana) because of its bending on to the object.
Then, just as a man, by following a snake that he has seen in his house, finds its abode, so too this meditator scrutinizes that mentality, he seeks to find out what its occurrence is supported by and he sees that it is supported  by the matter of the heart. After that, he discerns as materiality the primary elements, which are the heart’s support, and the remaining, derived kinds of materiality that have the elements as their support. He defines all that as “materiality” (rūpa) because it is “molested” (ruppana) [by cold, etc.]. After that he defines in brief as “mentality-materiality” (nāma-rūpa) the mentality that has the characteristic of “bending” and the materiality that has the characteristic of “being molested.”
Path of Purification, XVIII 2-4
Basically, it entails using the jhana factors and their physical support as a basis for insight - seeing them as impermanent, suffering, and non-self to the point of realizing the four noble truths.