[header] How to form concrete walls [title] Measure out the area of your wall and drive stakes into the corners. [step] Tie string between the stakes to serve as guides for digging. [title] Dig out the area of the wall. 
[step] You should dig down below the frost line, which is usually about 1 foot (or 30 cm) down. If building a retaining wall on a slope, make sure the bottom of your hole is flat.