Well, once you do the model, pick a vanilla weapon that would fit yours ( animations for example if you have done a bolt action rifle, choose the hunting rifle) then extract it with B.A.E, you´ll find it here on nexus, later, study the parts that compose the weapon itself, ( receiver, barrel, stock...) and detach your model to fit the parts that the vanilla weapon has (for example: vanilla receiver has a bolt and a receiver so you have to detach the receiver of your weapon into these two parts and export them as a single nif to get the receiver). Now follow this tutorial : https://www.youtube.com/watch?v=ppTXCJ0SZ88 and do the same with all the parts.