How to creat .stl model from DICOM USING 3D Slicer?

Dear Friend, wish you are doing fine.
I am working on a project in which I need to use .stl files from CT scan in DICOM format. So I need to change the format to. stl and creat 3d model how could this be done?
Also, I want to do a superimposition of two different times CT using 3D Slicer: I am using CMReg…Surface Regi… but I do not know why it is not working? I had the CT in good relation but the mesh formed away from both CT, and when I use Model to model distance the numbers wrong as showed by mesh statistics… Could you please help me in this?
Thank you again and wish to hear from you soon.

You will need to segment the image. See more information here: Image Segmentation — 3D Slicer documentation

Surface registration cannot be used for registering images, only two meshes, and only if they are initially transformed close to each other. See more information about applicable registration techniques here: Registration — 3D Slicer documentation