Depends on how low lying your garden is, and what type of soil, etc. Mine gets very wet because it's next to (and pretty much on the same level as) the River Trent. Unfortunately we get lots of gnats and mozzies because of it
If your garden is like mine, then you're probably better off laying a proper base...I think you need to take the turf off and a few inches of soil...then put down a layer of hardcore (rubble and broken bricks, etc)...then build a wooden frame out of timber to hold the concrete whilst it sets...pour the concrete in and wait for it to set...then remove the wooden frame.
I *think* that's how you'd need to do it, but I'm not totally sure...I bet you'd find instructions on a DIY website