You have to know the width of your menu to do it, and the actual classes/ids you’d use would depend on which menu you’re trying to center, but basically you need to make the ul
element a fixed width and give it margin: auto;
. Something like this (tested in the browser on my blog:
ul#menu-header {
margin: 0 auto; /* this centers the menu but only if...*/
float: none; /* ...the float: left is offset and...*/
width: 550px; /* ...the width isn't 100% */
}