Making something double-sided in Blender.

I’m making that hat in blender and I plan to export it for use in Gmod and whatnot. (The texture I can fix later.)
The left image is the top of the hat, the right image is the bottom. How do I make it so the hat isn’t invisible from underneath?

Well, there is a “make double-sided” function in Blender (either under Face or Normal), but that won’t work in GMod. You’ll have to duplicate the faces, then flip the normals. Or use the extrude tool.

How should I extrude it? Just select all the brim faces and pull down a bit with the extrude tool? This is really my first actual model and I’m not too farmiliar with the program. Thanks for the advice though.

That would do it. You may want to do the same to the inside of the hat part of the hat as well.

Oh, I’ve figured it out. Thanks.