5 to 1 sand cement then you want some plasticiser in it so it glides then just lay a wooden frame and fill it with the concrete and screen it level.depending on what weight will go onto you will have to have a solid base be it hardcore or just solid ground whackere plated down. hey presto
my dad is a landscaper and does that kind of thing all the time im not sure how much he would charge you to do it tho.
or you could pay me and ill do it