Setup within Kartris | Kartris User Guide

Go to Configuration > Payment and Shipping Gateways and click to edit Realex.
MerchantID - supplied by Realex. This is not the merchant number supplied by your bank.
SharedSecret - This is supplied by Realex. It is used in the generation of the SHA1 hash. If this does not match the one on Realex, transactions will not be accepted by the gateway.
ProcessCurrency - You must set this config setting to the 3 letter ISO currency code of your Realex account. For example, if your account processes Euros, this setting must be set to ‘EUR’. Setting this config setting correctly ensures that all orders will be converted to Euros prior to passing over to Realex. If you don’t set this (and your store supports multiple currencies), you will pass orders in GBP or USD to the Realex gateway, but it assume these amounts are in Euros.
AutoSettleFlag - Used to signify whether or not you wish the transaction to be captured in the next batch or not. If '1', then all the transactions will automatically be settled in the next batch. If '0' it means that you will manually settle transactions after the goods have been shipped.
URL - address of the secure payment page on the Realex site.
ReturnTSS - Use to signify whether or not you want to use Realex’s Transaction Suitability Score. If this is '1', six additional fields will also be supplied to Realex in every transaction (shipping code, shipping country, billing code, billing country, customer id, customer email). Otherwise, set to '0'.