The standard sizing gauge available these days is a ‘hole’ type of gauge most easily used for knitting needles. The DualGauge is a variation on this theme, however it allows you to slot in crochet hooks as well by sliding them in to a groove with the appropriate size marked on each.
So where do you measure? Technically, you should measure where the taper of the hook or needle is finished, on the shaft. However what’s really important to know is the size of your tool at the point where you use it, so always measure the working area for your method of knitting/crochet.
To find the corresponding size of your hook or needle simple slide it in a slot that you believe it could fit into. If the slot has ‘wriggle room’ move on to the next smaller slot until you find one where there is no room to move. Try not to force your hook or needle in to a slot as this could risk damaging both the gauge and the yarn tool.