Riyaz.
There are 2 parts to this: the payment details in your swap deal, and the configuration of your flow types.
1.) in the payment details of your swap deal you need to allow netting. This can be defaulted by the settings in the business partner (see Qadeer's post above), but the relevant parameters must be in the payment details of your deal.
In addition, of course, you must be using the same bank account for incoming and outgoing interest.
2.) the interest payment flow type has to be set to allow netting as well. That is in configuration, where you can set the flows to allow netting for incoming, outgoing, or both. This is usually already set to allow netting in standard customizing.
When you now post / generate the payment request, the payment flows will be receive the netting key at the level you indicated, e.g. the deal level. When you run F111, the payments will be netted as you would like to see.
I hope this helps.
Regards.
Dirk.