how do i activate a iPhone 6s without a sim card

You can't

You can not.

Why do you think you need to activate it without an SIM card?

See this thread, particularly the post by Lawrence Finch:

How to update iOS 9.3 without SIM card

An iPhone must have a SIM card in it to activate it. The SIM must be for the carrier the phone is locked to, if it is a locked phone, but it does not have to be a currently active SIM. Once the phone is activated the SIM may be removed

so do the sim card have to be on?