I don’t see many tutorials about rigging mesh for OpenSim. But, Hypergrid Business has a new one. See: Quick mesh rigging how-to. Of course the procedure will work for Second Life too. But, this is for ‘rigged’ mesh only, not fitted mesh.
While they use the Machinimatrix base avatar, you do not need AvaStar. However, not having AvaStar means using the parts of the model included for working with fitted mesh is awkward and not at all intuitive.
I haven’t played in OpenSim since OSGrid crashed. So, I am not sure whether or not we can use fitted mesh in OpenSim. So, this tutorial is more than adequate for most of those making clothes in OpenSim.