Assuming your using single shot AF and not servo... you simply align the center focus point with the eye, half-press the shutter and HOLD. This locks the focus once it gets a positive focus confirmation. Then recompose. The focus won't change as long as you keep the shutter halfway pressed. Focus and recompose.
FYI, it's pretty convienent when handheld, but two problems that you may run into with this method...
One, when you recompose, the focal plane shifts slightly so you are guaranteed to not be 100% on if you recompose. Fortunately, this only matters with very narrow DoF.
Two, when shooting off a tripod. Recomposing is a pain, sometimes you just want to set the focus and leave there without having to fiddle with the AF/MF switch on the lens. And not something you want to do if you are shooting HDR brackets and you choose not to or are not able to bracket automatically.
Personally, I use the backbutton focus and I choose the focus point manually so recomposing is minimal.