Step 1: Digging In...

Use a pole to establish the center of the fireplace. This will be a reference point so make sure to use a long pole. Use a string and some spraypaint to mark the edge of the fireplace.

Then you can start digging... To find the right height you can add sand and some cobbles. Make sure the height is approved by apropriate authorities (wife, girlfriend, whatever) and then make sure this stays where it is as this will be another reference point for you.

Step 2: Adding Gravel

Use at least 5-7 cm of gravel and stamp it firmly.

Step 3: Adding Sand

Then you can add some sand. Use your 'cobble reference' to determine the amount of sand. I suggest at least 10 cm of sand. Do not stamp it but make sure it is evenly allocated according to your spirit level.

Step 4: Laying Cobbles

Use your spirit level and your 'cobble reference point' to make another two or three reference points at the outer perimeter of the fireplace.

Then make the inner perimeter. Again you should use a string on the reference pole with your chosen radius.

In order to keep the right level at all times you should lay down a few 'spokes' of cobbles wich will turn out helpfull when laying the rest of the cobbles.

You should use a rubber mallet when laying the cobbles.

Step 5: And More Cobbles

Use your reference points and take it away!

I suggest you start from inside and out. Make sure not to add sand between the cobbles yet.

Step 6: Checking Your Own Work

Use something long and strait to check if you have done your work properly. As long as you did not add the sand you can easily move, lower or raise each cobble.