The general process for adding rules is described under Create a new rule - Documentation - Confluence (atlassian.net).
In order for the transaction distribution to work properly, make sure the corresponding rule is always placed last within its flow.
To apply weights, you must include a condition with the field txn_currency. The rule may include other conditions with other fields. txn_type field is disabled in the Field drop-down list if the weights distribution is applied. The rule will be automatically applied to payment and authorization transactions.
The conditions with logical operators In set and Not in set must not be used in the rule for transaction distribution by weights.
Click the Use weights toggle to the on position. In the fields below select gateways from the drop-down lists. Click +Add to add additional gateway fields if needed. In the Weight field type the desired weight for this gateway. Clicking Fill all empty will assign the value of the remaining weight to the corresponding Weight field.
The update file will display the date of the last rule update. If the rule is updated (gateways are removed/added, the ration is changed), the distribution restarts.