I found the metta and Karanya metta sutta's through google but they seem to be general discourses on metta.
You wont find in the suttas any detailed or specific meditation instructions for formal metta meditation practice the way that you would find for say anapanasati. They tend to be very general outlines and descriptions of practice rather than systematic explanations, though the Karanya metta is probably the most detailed.
The standard loving kindness meditation instructions that we know of today and that our most often taught these days ie. the spreading of metta first to oneself, then a benefactor, a neutral person etc. come from the commentaries most specifically the Visuddhimagga.
In my own practice I do not do much formal metta meditation practice but rather keep metta as a theme for contemplation and an attitude to be kept constantly in mind during my daily life and interactions with others.
