@mikkel would be the right person to tell you the exact steps (if you are working with Radiance). But typically, you’d have to start with making a new contribution to the exisiting recipes while following the existing code-style and syntax etc. In this specific case, you would be adding a new recipe to honeybee-radiance while utilizing the underlying libraries. The recipe would then have to be incorporated into the GUI via the grasshopper plugins.
Things are bit more sophisticated now with the new LBT plugins, but I got my start by asking question similar to yours around ten years ago.