Step 1 An ideal height for a raised vegie garden is about 450mm. If basing your garden around an existing brick structure, as here, demolish top courses using hammer and bolster until it is near this height.
Step 2 Dig holes at corners of garden about 400mm deep. Using power saw, cut sleepers to 750mm long and drop into holes. Fill bottom half of holes with rapid-set concrete. Make sleeper posts plumb and level with each other. If going off an existing structure, make top of posts level with that structure. Also make sure faces of posts are in line by pushing another sleeper against them until you have even contact at both ends. Fill rest of hole with more concrete and soak. Let set.
Step 3 Cut sleepers to fit between posts. Lay first sleeper down so top is 200mm below top of post, then make level. Predrill using 4mm drill bit, then screw sleeper to post. Place second sleeper on top and fix to post.
Step 4 Create a step to garden using sleeper offcuts. To support step, use a long piece at front and butt 2 pieces 130mm long on each side. Screw them together. Cut tread so it overhangs support by 20mm on front and sides. Centre this on 1 side of garden and screw through sleepers into step to secure.
Step 5 Lay lengths of sleepers as capping pieces on top of garden walls.