The reality of this process should be told and that is there hundreds of methods to create clothes and there cannot be said, to be one and only one correct method. Often you may combine different workflow methods and application / program pipeline configurations to get the job done and the best method is the fastest error free method that you have available or the method that gives you the best results. Zbrush is not a free program so maybe you like 3d coat as it is cheaper ... well then you will use a similar method in 3d coat. In order to make the upper body template that I used in zbrush ... I save off a copy of my female second life avatar after importing it into Avastar and blender first and after I saved a copy I then went to file/ export and I chose to export my models body as a .obj file (wavefront file) and saved it. The I opened up Zbrush and I imported the models object file into my zbrush program and I hid the hands, the lower torso as well and deleted the hidden, then I grouped the head and upper body separate and then I split the groups so I had the templates in two different subtools. This made it very simple to then dynamesh the body section and then I used the zremesher guide pen and I drew the flow of polygons for the topology with symmetry on and I then set the zremesher to adaptive, set my curve strength to 75, set my polygons count to around 5 or 6 K and zremeshed the upper body. At this point I took a look at it and the topology was not that bad so now at this point I have a template of the upper body in order to make my cloth with !!!
Seen here this example can now be moved into Blender and Avastar or I can texture a very high poly version, texture the surface , make a normal map and texture map in zbrush as well the UV's ... But that would be a mistake ... First I want to import this into Avastar as shown in the first picture and make the UV maps for it ... then take it with those back into zbrush as seperate copy ... then the version I have now textured and painted in zbrush I would then project all to hte version that has has UVS previously done ... This project all will be of the texturing and the painting ,,, from there I will now make the texture map and normal map from the copy that I created the uvs for in Blender and export all of it again back to blender. From there if I have all the graphic appearances correct including the maps and as well deleting the high poly levels ... see now the maps will still work as they are made for the low or high poly version, yet these maps make it so you do not need the high poly version ... Remember some maps from zbrush may need to be flipped before uploading or exporting to both Blender or to second life ... Yes we are now ready to avastar to rig and weight this version ... Again I am giving you the work flow this is not a 3d zbrush and Avastar, Blender tutorial. There are so many ways that a piece of clothing can be made and this is just one method! Most of all have fun ... yes you may get stuck at times ... In this case take a look at tutorials and videos about the process and other workflows for 3d animation, texturing and modeling.