How to build a raised bed

1Clear and level the site for your raised bed, then spread a layer of hardcore. Make sure it's about 8cm deep over the entire area.

2Compact the hardcore using a wacker plate. This machine will flatten and level the hardcore, ensuring the footings for the bed are more secure.

3Using the line-marking spray paint, mark out the shape of the bed. For straight lines, tie a piece of string to two wooden pegs, secure the pegs in the ground, pulling the string taut, then spray along the string. For curves, tie a piece of string to a pole, pushing it into the ground at an equal distance between each end of the curve. Pull the string taut and mark where it meets one end of the curve. Walk the curve, keeping the string pulled tight, and spray. Leave the pole and marked string in place.

4Make a mortar mix - five parts building sand to one part cement. Starting with the sprayed straight edges, lay the first course of blocks, placing them on their longest end, on a bed of mortar 5cm deep. Run a length of string between two pegs to help keep the blocks in a straight line. Check the blocks are level with a spirit level. Lay the second course of blocks, staggering the joins, using a 2cm deep bed of mortar. Use a hammer and bolster chisel to cut blocks in half for the ends. Check the levels again.

5Lay concrete blocks, short end down, on a 5cm bed of mortar to make the curved edges - this is the same height as two courses of blocks. Check curved line match up to the mark on the string tied to the pole. Lay the ends of the curved edges first, working towards the centre.

6Carefully slot in the last block, cutting it to size if necessary. Make sure all edges of the bed are level on the vertical and horizontal, and tap them straight with a rubber mallet if necessary. Leave the whole thing to set overnight.

7Make a render mix of five parts sand to one part cement, adding a dash of washing-up liquid to make it easier to apply. Using a rendering trowel and hawk, juggle the mixture like a pat of butter between the two tools until you get a really smooth consistency, then apply the first coat of render to the blocks. This should be 5mm - 10mm deep, and don't worry if it's quite crude. Only render halfway down the inside of the bed, as the soil will cover the rest. Once you've finished, score large diamond patterns into the surface using the corner of your trowel. This scratch coat will set overnight, then you can apply another, smoother coat on top.

8Apply a second coat of render. Before it sets completely, gently rub it with a piece of foam or sponge. This will smooth out any imperfections, giving a consistent finish over the whole surface.

9Leave the bed for at least a week, then finish with masonry paint. It's essential to leave it this longs as the firmer the render gets, the stronger the structure will be. Once the paint is dry, fill the bed with topsoil, stamping it down as you go. Then it's time to get planting.