How do I attach a mesh to a rig? I've spent hours trying to use the Automatic Weight option in Rigify, but the left forearm of my mesh deforms(tried merging verts and scaling).  Everything looks fine until I parent them.  I want to keep the generated rig, but are there any alternative methods to attach it to the mesh?
Have you tried the Automatic weight?

Did you do the retopology?

The first time I tried doing Rig, it didn't work until I learned about retopology, wich is (im not that sure) putting less ''points''. I don't know how to explain that correctly, you should watch the video of FlippedNormal on retopology for begginer.

If you already learned about this, maybe look if you have only square, for me when I have triangles it dosen't work.

Hope it will help. Good luck