After you have blocked your hat, you will cut the millinery wire in the appropriate size of your brim edge. (You can use a measuring tape and measure along the edge if that easier for you). I am using 21 gauge in this video.
Some milliners use millinery wire connectors but I find tying the ends of the wire together with thread works just as well. Simply wrap with thread (many times) and tie.
Next, using the zig zag stitch on your sewing machine, you will sew the millinery wire onto the edge of your hat. My machine has three zig zag size settings. If you also have that option, the middle size gives the perfect zig zag width.
Next you will sew on the gros grain ribbon at the edge of the brim to hide the millinery wire. To do this, first you will fold the ribbon in half and curve with the hot iron to give shape. You must use gros grain ribbon to do this, other ribbons does not have the curving ability. (You could try using bias tape, if that is more suitable for you. Personally, I did not like the finished look.) Then you are ready to sew. The ribbon will be impossible to sew neatly if you do not curve the ribbon prior to sewing.