When setting up a scene to design clothing in blender the first thing you do of course is import the avatar as a base. And you will notice all the bones are positioned vertically. Which looks wrong. The armature/skeleton bones are usually aligned inside the mesh like an actual skeleton. You first reaction might be to fix the bone orientations. Under the FBX importers armature section you can enable Automatic Bone Orientation as shown. Which will give you something like this. See, all nice and aligned. And a disaster waiting to happen. NEVER DO THIS. Import the model unaltered. As shown in the first image. Then build your mesh around it. Your mesh is all finished and ready to export. Select the skeleton and press ctrl-a. Then apply the scale so X, Y, and Z are all 1. Then select the skeleton and your mesh and export. Export as DAE for best results. If you export as FBX it will cause more issues. Now import your DAE in to your unity editor. Drop the object in the hierarchy and expand it. Double check all your scales and rotations. Your scales should all be 1 for the main object the reference and the main object mesh. Zero up the X rotation on the main mesh to -90. Apply your clothing settings and prepare the mesh. It should generate deletion zones and tucking properly. Apply virtual good component and upload and you should be set.