craftgrrl | Cushion cover with zip closure

I went charity shopping this morning and scored a couple of retro duvet covers and an unusual Irish linen tea towel. They're begging to become cushion covers, but the last time I tried to sew one with a zip closure the ends of the zip ended up looking rubbish.

Can anyone point me to an online tutorial (with pictures) that clearly shows how I should be dealing with sewing the edges of the zip to the two fabric panels (front and back of the cushion)? Obviously sewing the main part of the zip to the panels is easy, but when it comes to the edges they always end up sewn to either the front or the back panel, giving an uneven look.

Thanks so much guys!