The battery parasitic drain test procedure
Without disconnecting any terminal of the battery, clamp the negative wire of the battery and set the clamp meter in the range to measure current with the jaw of the meter and begin to read the current displayed.
Note: If the battery of the car is seating in the boot, you can still clamp it there taking note of the little current draw from the boot lights. Alternatively, there will likely be a positive battery cable in the engine bay that supplies the kick starter power.
If there is excessive current draw, you can begin to pull fuses to identify which one is causing high current draw. Once that fuse is identified, you will need to know the accessory or accessories that fuse is protecting. Check the owners manual of the car for this information.
The clamp meter can also be used to test individual circuits like electric power steering, radio, electronic control modules and so on.
Note that some cars have multiple fuseboxes in their engine bay, cabin and boot areas.
Ground connections should also be checked for Parasitic current drain because poor contact points can create a region of high resistance which can cause current draw.
Major power cables from alternator to battery and from battery to kick starter should be checked for parasite drain. This can be done effectively through voltage drop test that I will treat in another different post.
If parasitic battery drain cannot be traced to any accessory, ground or major power cables, it is high time to check the alternator diodes because faulty diodes allow current to leak in the opposite direction. A diode when operating normally allows current to pass in only one direction except a very tiny amount of reverse current that it can pass. Do not let the reverse current confuse you. Just ignore it and know that a diode allows current flow in one direction. A faulty diode will allow current to flow from the battery to the coil of the alternator causing parasitic current drain.
You need to have a good information about the car that you are testing before you will know what level of current drain is parasitic. Some cars with keyless entery can have their modules searching for the key signal and these modules draw certain small amount of current which can be mistaken as parasitic drain. That is one of the reasons why some of those cars are equipped with large batteries like 100 amps battery. It's good to consult repair manual for information on current draw of the different accessories if provided.
You also need to ensure that the charging system of the car works well and fully charges the battery because a battery that does not attain full charge drains faster.