Make sure that the trough is level and has adequate drainage holes. Put coarse drainage material over the drainage holes and over the base of the trough to a depth of about 5 cm. You can put anything from pieces of broken pots to bricks or other rubble, any of these are suitable for a good drainage.
Top the drainage material with a suitable free-draining alpine compost, adding it in layers and making sure that is firmed in thoroughly, especially in the corners and around the edges. Fill to within 2.5 cm of the top to let some space for a safe watering.
Decorate the surface with attractive pieces of rock using your imagination. Angle the pieces into the compost making sure that about one-third of each piece is buried to create a mini-outcrops that will provide various pockets for planting your chosen alpines.
Plant the selected alpines in the trough by scooping out holes to take the rootball of each in turn. Firm the plants in but avoid the temptation to over-form. Place the larger and central plants first, finishing with smaller ones around the edges.
Finnish the arrangement with a top-dressing of coarse grit or rock chippings, pushed well under the top-growth of each plant. Keep well watered at first but avoid wetting the leaves unnecessarily. Once established, the trough should only be watered occasionally.