They just screw onto the top of the presta valve (which is left open) and some have a hex slot in the top to tighten it up fully.
You can put a small dab of superglue on the open valve to prevent it sealing whilst you are riding along.
Alternatively with heavy extenders you can screw them on as above and then inflate the tyre, remove the extender and lightly tighten up the valve with a needle or screwdriver - but you need to remember to take the extender on every ride.