All year round
Mark the bed shape by laying the blocks on the ground. Dig a shallow trench, 30cm wide and deep, for the foundation the blocks will sit on, and fill with a 20cm layer of rubble. Tamp down thoroughly with your heels.
Top up the foundation with a 10cm layer of Postcrete and spread out using a shovel. Use a spirit level placed on a straight board to check the surface. When it's thoroughly level, water the Postcrete thoroughly.
Set the blocks on a bed of mortar. Start building at one corner of the foundations, using mortar to stick the bricks together. Build the other corners level, then the walls, checking they're straight with a spirit level.
This raised bed is made from concrete blocks and topped off with timber sleepers to provide extra seating space.
Once the blockwork has set (after 24 hours), make up the render (two parts sharp sand, 2 parts building sand, 1 part mortar). Add 600ml PVA adhesive per barrowful. This makes the mixture easier to work with.
Wet the blocks using a watering can. This stops the dry blocks from sucking the water out of the render. Apply 5-10mm of render using a plastering trowel. Keep wetting the blocks so the render goes on, and stays on, easily.
Once the render is fully dry (after a further 24 hours) make the edge with half sleepers. Secure them by screwing offcuts of sleeper inside the blockwork. Get someone to hold the offcuts in place while you drill.
Paint the render using an exterior-grade masonry paint (this is the best type for using on rendered blockwork). A litre of paint will cover around 6 sq m of wall. You may need two coats.
Fill the bed with a soil/compost mixture such as blended loam or make your own mix using 2-3 parts garden soil, 1 part sand and 1 part well-rotted compost. Add more sand if you have clay soil.