To build a garden retaining wall, mark out the wall with builder's sand before digging out the foundation. Remove the top soil and place it to one side in a heap. Cut a number of wooden pegs to the same length about 20 to 30 centimeters and hammer them into the base. Set pegs to the same level at the top. Dig out the correct depth of the foundation and gauge down from the top of the peg to know how much you need to dig; to form the basic foundation depth. Set up a series of wooden pegs in the base of each trench to the height you wish to lay the foundations and level each one with a straight edge and spirit. Using a concrete mixture 6 to 1 ratio, pour the foundations and level using the pegs in the base of the trench. Once the foundation is dry lay the first layer of blocks or bricks using a rear skin of concrete blocks for a retaining wall faced with natural stone, mock stone brick or wood cladding. Laying the blocks in your wall should be done using mortar. Lay both ends and spend time making certain they are true and level. Using a builder's line strung between the two, you can lay the remaining blocks or bricks using the line; being careful not to distort it by touching it. A good looking retaining wall can turn your idle back yard into a safe, level area that you can actually use.