To create an ad hoc (IBSS) network, just pull down the Wi-Fi Menu Extra to "Create Network…". This is probably all you need. The Mac won't actually be acting as a true AP; it will just be a peer in the ad-hoc network. Also, the Mac won't be acting as a default gateway or DHCP server or anything, but it'll give itself IPv4 and IPv6 link-local (a.k.a. "self assigned") addresses, as will your iPhone. You don't need a DHCP server if all your devices know how to self-assign unique addresses like that, you don't need a default (or otherwise) gateway if you don't need this network to connect to another one.
If you really want your Mac to act as a true AP, default IPv4 gateway (a gateway to nowhere), and DHCP server, then go to
System Preferences > Sharing > Internet Sharing and choose to share connection "From:" <any interface other than Wi-Fi>, "To computers using:" Wi-Fi.