You don't really need the membrane, but if you want to use it, put it beneath the sand layer. The membrane will stop most plants coming up through it (if its thick enough, the ordinary, thin, fabric like black stuff won't do the job, you need a heavy duty one) but won't stop, for instance, heave from nearby tree roots or really pernicious 'weeds' such as japanese knotweed or bamboo. If you're mortaring the joints between the slabs, do that as well as you can, because a poor mortar mix or a bad job means weed seeds will blow into the cracks between the slabs, germinate and grow, with or without membrane.