Its really quite easy. I have built two at different homes, the first was built into the ground and took about three weeks the second took four hours to build and a further day to finish off. The easiest is to buy cotswold bricks which can lock together (no need for cement). Make sure you have a solid base. Build to the height you want and then put liner in making sure that liner is on sand to eliminate stones, sharp edges etc and then finish by cementing on a top wall of flatter cotswold stone. A compamy called Torridon make similar curved bricks but can be expensive. Local builders merchant is bet bet for better range