These things you will need:
A Bosch Remote Key
An OPCOM scanner
Remotes you can get from ebay or your local Scrap yard. Transponder Chips are identified on the chip itself, and assuming it is not a programmable one (cloned by a key cutter). OP COM you can get from Chinese sellers. Mine is from obd2diy.fr with free shipment.
Fire up you computer with OP COM and launch OP COM
Plug in the OP COM and Turn on the Ignition.
From the Main menu select diagnostics (assuming you have used OP COM and configured the settings before)
Automatic does often work, but it's usually slower than just picking the year and Model.
Select Body Module, then Immobilizer.
Go to Measuring Blocks and you can see what transponder is in use and how many are programmed (You may have 2 keys but only 1 transponder ID, if you've had one Cloned at a Key cutter)
For Immobilizer programming you need to enter the Security Code from your car pass.
You can then go to Special Functions and Erase Transponder Keys. NB: This will erase all keys and may not be necessary if you have all the keys that are programmed.
Once you've erased all Transponders you can turn the ignition off and on again. The EM light will be flashing to show no valid immobilizer signal received.
Now go to Programming. Not sure the difference between Program Transponder Keys new and old, but if new doesn't work tries old.
Now you've programmed a couple of immobilizers, you can view them on the Measuring Blocks option.
Turn the ignition off and on again to check the EM light is not flashing.
If your car has an Alarm then go to Anti Theft Warning, if you don't then go to Central Locking (The programming procedure is the same, but you have additional options with the alarm)
Go to Measuring Blocks, you can see what remote was used last, and the previous one.
Go to Programming, and if necessary Erase Remote Keys (This will erase all remote keys)
Go to Program Remote Key, then click the buttons on the remote you want to add (usually works first click)
If it doesn't work, make sure you have the right type of remote as mentioned at the top of this page.
Program another key if you like, and then you can view them in Measuring Blocks.
You do not need the security code for Programming the Remotes.
Hope this information is useful.