Open an existing iMovie project (these are in the ‘All Projects’ section in the Sidebar), or create a new one.
Make sure you have a few clips in it before you get started. Don’t forget that although projects must be saved with a specific event, you’re not limited to only use clips contained in that event, but can pick and choose from any footage stored in your library.
Read:
iMovie versus other low cost (or free) Mac video editors

All available titles can be found in the Content Library, which is located in the lower part of the sidebar. You can also access them by going to Window > Content Library > Titles, or by using the Cmd-2 keyboard shortcut.
Once chosen, the section usually reserved for displaying a selected Event’s clips is replaced by a series of title thumbnails.

Most titles are animated, which isn’t something the thumbnails show you. However, if you place the cursor over one and move it from left to right, you’ll be able to see what the animation looks like on iMovie’s main preview section.
The preview won’t be superimposed over whichever clip the project’s playhead is over, it’s just the text over a black background.

To add a title to your project, drag it from the Library on to the Timeline section.
Doing this transforms it into a thin purple banner. As you drag it left and right, you’ll notice yellow vertical guides appear in particular locations.
These are designed to help you align your title to the start, middle or end of a clip, for example.

The title has a little hook on its left that’s bound to whichever clip it’s hooked to.
Move the iMovie clip to another location and the title comes along for the ride.
Your title can also span multiple clips and you have complete control over how long it should be – move the cursor to one of its edge to turn it into a resize tool.

Click somewhere over the title so you can view it in the main preview section, then double-click on the placeholder text you can see there. This should make that text instantly editable.
The more you type, the smaller the text becomes so keep your titles short as each text box limits you to a single line.

Click the Adjust button, and you’ll find a series of tools you can further customise your clips with. The one you’re looking for is represented by a ’T’. Click on it to access mode options.
Sadly, not all titles are customisable and if you choose one of the first 16, everything will be greyed out.
Scroll down your Title Library and see whether one of the latter ones is to your liking.

Drag that new title from the library on to the existing title in your project. When it turns white, release the mouse button. This replaces the existing title with a new one, preserving its original duration.
If the new title has two text boxes and the previous one had one, the text box will be saved. But if you go from two into one text box, you’ll lose some text.

Go back to the text adjust tool and you’ll see more options. The font change is specific to each text box – you can’t apply different fonts to each letter, but you can apply italics, bold, outline and different colours to individual letters.
Select ‘Show Fonts’ to reveal Mac OS X’s traditional Font Palette. This will give you access to all the fonts installed on your Mac.

Scroll to the top of the Titles Library to see a section called ‘Current Theme’. If there’s nothing there, click on ‘All Projects’ in the sidebar.
Next, click on Settings (top right), then on the Theme button.
Choose a theme, then click on Titles Library.
You’ll see theme-specific titles there, which you can use to help give your project a unified look.
Also see:
Edit a movie using the Precision Editor in iMovie for OS X
How to use effects in iMovie for OS X
Add titles to your home movies using iMovie for Mac OS X