The old iPF app no longer works. You will have to use either the site or the Tapatalk app.
The site works pretty much the same on the iPhone as the iPad, except the menus are more cramped. Tap the "..." item if you don't see the choice you are looking for. It is a bit easier to navigate in landscape view.
Tapatalk is probably the better way to go on the iPhone.
After you have added iPadForums to the app, The principle is the same. Navigate to the forum area you want to post in, and look for the option to start a new thread. In this case it is New Post, and can be found under the '. . .' icon at the top right; when viewing the list of topics in a forum area.
I can go into more detail if needed.