How to use call recording on iPhone 6?

If you mean recording a phone call, you cannot do this directly - Apple does not allow access to phone audio for security reasons.

There are some apps that do 3-way conference call where they record/transcribe your conversation, but you pay a fee for that service. If they say it's free, be concerned how they pay for the phone lines you call in to, storage of your conversation, and offering to send you a copy since those all have a cost - ie: selling information gathered from your call, etc.

So, assuming you can legally record the conversation (some states/regions do not allow recording without consent of both parties) the easiest way would be putting it on speakerphone and recording with a different phone/computer/voice recorder, etc.