you go to properties, all the way down. Then you click surface imputs or surface i dont know, and then you see frontsurface, topsurface etc and they all say smooth. click on ‘smooth’ at the surface you want it to be on (in properties) and click weld.
To create most types of weld(s); two parts need to be touching. Once you’ve done that, you’re able to create a weld. Whether you want to use a Weld, WeldConstraint, or anything in-between. If you want to do more advanced types of welding; definitely do more research into the types of attachments.