After scaling and positioning your clothing mesh, begin applying sculpting detail to add cloth and fabric detail to your mesh. While there are many ways to sculpt an object in Blender, this tutorial primarily uses the Elastic Deform, Inflate, and Clothing tools to make your mesh look more like realistic clothing.
To add clothing details:
With the shirt highlighted, switch to Sculpt Mode.
Disable X-Ray mode, if required.
Enable X Mirror to perform symmetrical edits.
Select the Elastic Deform tool at .5 strength to stretch parts of vertices to completely cover the mannequin.
Use F to change the radius of the brush.
You can hide the mannequin to access hard-to-reach areas.
Select the Cloth tool.
Disable X Mirror. The cloth tool can produce unexpected results with symmetry enabled.
Using the Cloth tool, click and drag on your mesh to add a cloth-like surface to your mesh. Adjust the settings to change the strength of the deformations.
Using the Elastic Deform, Inflate, and Cloth tools, make the final adjustments to your mesh so that it sits on top of the mannequin with the final desired shape.