Brick planters are an ideal place to grow plants that need different soil conditions from the ones in your flowerbeds. A raised planter is also easier to reach if you find it difficult to crouch or kneel at ground level and is just four low brick walls built in a square or rectangular shape.
Like any brick or stone wall, a planter needs a solid foundation. But rather than digging separate footing trenches, it's easier to lay a 150mm-deep concrete slab base under the planter that extends 100mm beyond each wall.
Lay the first course of bricks on a 15mm bed of mortar, and use a long spirit level to check they're level.
To make sure that water drains away easily, leave a series of gaps or weep holes in the mortar in the first course of bricks. One every second or third brick should be plenty.
Lay the remaining courses of bricks. A good height for a planter is about 600mm (any higher and the walls might be pushed out of shape by the soil). Finish off by laying coping stones, then let it dry for at least a week before you plant anything.
You can stop your plants becoming waterlogged by filling the bottom third of your planter with gravel or crushed stone. Lay a sheet of permeable geotextile fabric on top, and then fill the rest of the planter with a mixture of compost and horticultural grit.