How to make a homemade tzatziki?

I'm making gyros for dinner tonight (mmm!) and have everything I need except tzatziki sauce. I couldn't find it premade anywhere in the store and I couldn't find other ingredients to make tzatziki (this store does not carry Greek yogurt or a few other ingredients that I use), so instead I bought a bottle of "creamy cuber" dressing. It's not Ranch dressing and it's not Green Goddess (whatever that stuff is), it's just cuber dressing. Can you help me 'fix it up' into a reputable tzatziki sauce? Thanks in advance!