When I learned to purl, I practiced by purling every row (as opposed to knitting every row). It ends up looking just like knit garter stitch. As I went, I could see the difference in my stitches; they tightened up and became more uniform. It just takes lots of practice. I didn't start out making anything; I just wanted to practice. I think I actually ended up frogging my practice piece and reusing the yarn on an actual project.