Miriam the Brother manual instructions have you set the stitch length, then turn the stitch width knob back and forth while sewing to make whatever different patterns you want. It takes some practice to get any consistency.
I just did some decorative stitching down the middle of 115" of cotton strapping for a bag I made. I used my Brother Selectomatic, which has several built in stitches, but it has a mind of its own and randomly changed the length of the design on me several times. I was cussing up a storm but finished it, I wasn't ripping it off and starting over. It looks like I did it manually on a plain zig zag machine. Actually it looks worse but it is what it is.
Cari