mesh for noob

Hi,
Coming back to second life after 4 years and I found news like model or mesh.
I read something about import .obj and save them as dae but when I try I am not able to do something that is reimportable in sl.

Is there someone tha can explain me the exats steps and parameters for importing .obj file in blender and export it in dae for upload in sl?

I'd want to copy some clothes in particular.

I know that import dae in sl cost lindens but cost less than cost of cloth

thanks