First- what kind of button? Is it the kind with one hole behind it in a shank, is it two holes through the front, or four holes through the front?
If it is a shank button, just carefully stick your needle through at the same spots every time you go in and out of your fabric. You want enough thread wrapped through so that it's sturdy, but going overboard with stitching makes it look sloppy. You can't avoid tying a knot, but make it as close to the stitching as possible. I tie three square knots on top of each other (they look like one knot).
If it has two holes, making a shank will hold the stitching against the inside of the bag, which will be neater. How to make a shank: attach the button to the bag as you would usually do, BUT don't pull it tight against the fabric, leave about a quarter inch of space between the outside of the bag and the button (it will look like it's dangling and that's good). Push the needle and thread from inside the bag to the outside, but don't go through the button, instead wrap the thread around the loops beneath the button a few times. This makes the shank which is like a stem for the button. Push the needle back down through the inside of the stem (that's important or it won't look right) and fasten off on the inside of the bag. One bonus to a shank- your material won't pucker when you close the button, because it's not pulling on the fabric.
If you have a four hole button-the only instruction I have to add to the two hole button directions is this- decide whether you will sew the button in an cross or parallell lines-- X or ll --then stick with that shape so that it will be neat.
If this seems confusing, I recommend "Sewing for Dummies" or "Everything Sewing" in order to see pictures. Or you could google "sew a button" and see what you get. Good luck!