Initially make sure there is definitely not an earth rod present as it can be tedious installing a new earth rod, depending on ground conditions.
Sidney has given a lot of useful information.
The most cost effective method for small installations is using a copper clad steel extendable earth rod, or two, or three or more!
You are looking to get the lowest resistance you can with the most practicable installation, very variable around the country and even if the geology is different next door!
5/8" earth rods are the most commonly available sensible choice, the 1/4" rods bend easily and are not connectable to give more depth, thicker rods or all copper rods are not going to make a significant difference to the resistance on a small installation.
If the insertion of the rods is easy then going deeper with longer coupled rods is generally more stable than having multiple earth rods only 1200mm deep, though if you only have a thin layer of earth you do not want to be trying to hammer through rock. If the ground is really bad you can drill with a long SDS drill bit first but make sure the hole is smaller than the earth rod so you get a better contact with the ground, best avoided if possible.
You will need a means of being able to hammer, either with an SDS drill on roto stop or by the good old sledge hammer, the earth rod in without splaying out the end.
I tend to use a coupler on top of the rod with a 5/8UNC bolt in the coupler, but the coupler does take some strain. For the SDS I have a socket for the bolt welded to an SDS end.
Different earth rod clamps are available, you will probably get the best connection with a lug connection to a bolt, but I often use the clamps that allow the cable to be clamped with the rod (the screw being always against the rod not the cable) as it is quicker. But if you are using a thin earth cable it may not hold.
Ideally you want an decent earth pit to protect the connection and yet still make it accessible for testing. Unless there is traffic driving over it a plastic one should be fine but if there is then a concrete one. If you are saving costs then the small plastic boxes can be used, the little round ones look a bit rubbish. The earth rod should be labelled with "safety electrical connection, do not remove" (for some reason, regulations or something!).
Remember that the current taken will be low so a relatively small diameter (not less than 2.5mm²) cable can be used, but it is often easiest to use the 10mm² you have in stock, 16mm² if buried.
