You definitely need a zipper foot, if you tried to do it without you could end up breaking the needle and bits flicking up.
It is quite easy with a zipper foot, but if you're new to it, I'd suggest pinning it in place first, then tacking it with a contrasting colour thread.
Once you've got it in place the zipper foot glides in a straight line about 0.4 -0.5 cm from the zip, ie in the middle, while you guide the fabric through, gently but firmly.
Have a close look at zips in shop bought garments. It's usually best to double or triple stitch across the bottom, especially with trousers, as this is the place where zips get the most 'stress' when opened and shut, if you see what I mean.
Happy crafting!