Learning to Design Mesh Clothes
Off Topic
There are a few other things I would add to the process, i.e. once you get the rigged mesh imported into SL you will have to texture it (shade maps created in your mesh tool may be incorporated into this as well) and you will probably want to include alpha layer (s) to mask the body underneath so that it doesn't bleed through.
Some other thoughts.
Rigging/Weight Painting: check out Avastar to help with this process. You can also hire people to do this step for you.
Sizing is another area to explore. There is Standard Sizing as well as Fitted Mesh (and/or a combination of the two) for your basic SL Avatar. There is also sizing which relates to specific custom mesh bodies.
Layering mesh clothing is an art in itself.
Long full skirts can be done in mesh but the movement of them always seems to disappoint. Perhaps it's because I'm just use to flexi skirts but I'd love to see an example of a mesh skirt that really flowed. Does anyone have one?
Getting started: As you can see, there's a lot involved in the creation of mesh clothing and it can appear to be overwhelming especially if it is ALL new to you. My advice is to start small, perhaps by purchasing a ready-made full perm mesh item of clothing and learning how to texture it to begin with.