We meet in front of the stairs leading up to the Parcel Yard (next to the Platform 9 3/4 Harry Potter shop inside King's Cross Station) The guide will be standing there holding a sign or waving a wand.
After your walking tour ends, your guide will give you your tickets to The Shard Viewpoint. It's just a few minutes walk away. This part of the tour is unguided so you can enjoy the viewpoint at your leisure.