First of all if you're new to grado mods you should look through these threads:
http://www.head-fi.org/forum/thread/508459/sr60-mod
http://www.head-fi.org/forum/thread/119314/post-your-grado-mods
About the grills.
I heated them up with a hairdryer before i removed them. This melted the glue so there weren't any plastic left after the removal. If you already have removed them, and there's still plastic left, you could try to warm them up again and remove the remaining bits, or you could use a dremel or something similar.
You should use some wire mesh made of stainless steel. If you live in europe I know a great website that sells it:
http://www.inoxia.co.uk/rw/category/9.aspx If you live in America there's plenty of websites that sells it, just google "wire mesh"
About the sandpaper.
By writing 60 to 1200 I mean the sandpaper the grit number of the sandpaper i started with, and the one i finished with.
Thank you for your questions it's only fun to answer them.
Happy listening
Chippechanga