Pick a level spot with at least 6 hours of direct sun. A common beginner size is 240×90cm — wide enough to reach the centre from either side. Mark the area with string.
Use 38×140mm treated pine sleepers. Cut two 240cm lengths (long sides) and four 90cm lengths (short sides, for 2-high walls). For 2 high walls you need two of each.
Stack and assemble the timber into a rectangular frame, two boards high. Drill 10mm pilot holes and join corners with 120mm timber screws. Check for square by measuring diagonals — they should be equal.
Cut four 50×50mm timber stakes to 45cm length and sharpen one end. Drive a stake into the ground at each inside corner to anchor the frame and prevent bowing.
Line the inside walls with cardboard to suppress weeds, then fill with a mix of 60% topsoil, 30% compost, and 10% horticultural sand. Water thoroughly before planting.