I put some jingle bells on a long string by our door. Every time I took Toki outside to pee, I'd take her little paw and hit the bells with it (sometimes I'd just kick the bell with my foot on the way out). After a couple of days, she got the hang of it. Here's a video on how to teach it
I kinda skipped the step where I rewarded Toki for touching the bells before letting her out but I like Zak's overall philosophy on dog training!